This is probably one of the most basic ones on here. There are two kinds of ramps. Wide ramps, and narrow ramps. There is also spam ramps, which is probably the easiest of all, but least BP efficient.
Wide ramps are simple. First, look where you want the ramp. Then, turn exactly(or just about) 180 degrees around. Now, DO NOT MOVE YOUR MOUSE. The only thing you will be doing is clicking and walking. First, place 3-5 unconstructed chevax-de-frise or something like that. Next, step back a little, so you are right on the edge of the chevax. Now, place a plank. Move over a tad. Place a plank. Continue this until you are done. Bam, you made a short, very wide ramp. Some good uses for this? Elevated spot on a sandbag line, watchtower for officers to see over smoke from infantry fire or cannon fire, or accessing things barely out of reach. (note, the chevax can be replaced with constructed sandbags, among other things, but chevax are the easiest.)
Now, we have the narrow ramps. These require a lot of finese, and skill with the m&b character movement. First, look at where you want to go. Next, turn 180 degrees. Then, place a plank. Now place an unconstructed gabion. Repeat until plank is at sufficient angle to reach whatever you are trying to get too.
Spam ramps. These are used as bridges, Getting to really high places such as rooftops, or just getting really high. Unfortunately, they eat up BP like a zombie in a neurology lab, so if you only have 50 bp, don't expect to get very far. This ramp is super, extremely, incredibly simple. Usually done with chevax de frise, you just place one, inch forwards a little, place another, and repeat. Be careful, though, because most servers(and people) will probably consider this trolling and wasting BP.

There are two kinds of towers, one that is fairly basic and probably won't get you kicked, and one that you will want to make sure everyone, the admin, players, EVERYONE, that you are not trolling.

The first is pretty simple, and also highly effective. First, make a wide ramp, from the ramp section. Then, place sandbags in front of this ramp. keep building and stacking these sandbags until they are 3 high. Make sure they aren't too close to the planks, though. If needed, destroy the chevax de frise. After the sandbags are 3 high, add one more to the top, and then put some sandbags on the sides of the ramp. Bam, you have yourself an impressive looking elevated spot, its basically just a less exposing ramp, but still can be useful.

This second tower is very, very difficult to make. It takes a lot of wiggling and strangeness, but can be very high. First, do 3-4 plankjumps. These are just jumping, then midjump placing a plank, then jumping again, and placing a plank, and so on. Now, you should be off the ground. Next, place unconstructed sandbags at either end of the plank you are standing on. Step onto a sandbag. Look at the other one, and start trying to place a plank. You might have to wiggle a bit backwards or forwards, but eventually you will place a plank. Now just place planks all along the sandbags, similar to how you made the wide ramp. This will create a platform. Now, jump off. You will need to use some complex ramping, but make a ramp to go up to the platform. Bam, now you can just add some salad toppings like sandbags, heck I have even seen someone put a cannon on this thing. This is especially useful against AI, because AI isn't smart enough to go up the ramp.

Technically there are 3 kinds of towers. We forgot, you guessed it, the spam tower. See a recurring theme here? Well let me spice it up with something a little different. You use probably the chevax de frise. You place them one on top of the other while walking on them. Not BP efficient, blah blah blah. Pretty much a spam ramp minus the horizontal movement.

What I mean by this, is how to build things that do well against the AI in mount and blade. Obviously, there really is only one tactic against players, which is either spam, or kill them before they reach the fort.

The basic idea behind most bot tactics is that bots can't jump, people can. Build things that you can't walk over, you have to jump over. Bots don't attack cover unless players are directly behind it. This means placing fences or other objects in front of your main bullet cover. Be aware, however, that on most maps, the bots arrive pretty quickly. Usually the sappers who build cover die from cavalry riding up and killing them. Make sure to keep your head on a swivel, be careful.

Cover, is very tricky. There is somewhat of a schism between two kinds of excellent cover, sandbags stacked on sandbags, and earthmounds with sandbags on them. From personal experience, the first works better, and is quicker to set up, simply because with earthmounds, it usually takes several tries to place the sandbags even CLOSE to being level, whereas with 2 sandbags on eachother, it nearly always takes just one try. Plus, bots, even cavalry can't get over sandbags, in the event that one of your fences breaks. Please note, people. We all know that sapper who places a bunch of stakes in front of their fort. Don't. Its stupid. They don't even block bots as well as fences do, they take just as much time to build as fences do, they don't hurt bots, players have trouble jumping over them, they cost way more buildpoints because they only block about a third of the area a fence will block... just don't.

Some other neat things to do against bots:
-Tnt. Careful, spamming tnt can get you kicked/banned on many bot-related servers, but just 3-5 tnt can create a huge feed. What you do is you build some kind of box, quickly too, and in front of your team. You wait for all the bots to gather around the fort, (keep an eye on the durability of the walls) then you blow the tnt, which you would have placed all next to the fort except for one. Make sure that they are close enough to "carry" the explosion.
-Funnel. This is really, realy, really, really, difficult to do, and requires some extreme teamwork. Basically you will use fences to "funnel" bots into a killzone, some place where they are surrounded on all sides. This requires immense amount of sappers to build it quick enough, and a lot of coordination. Some other uses of it could be to control the flow of bots, so rather than have the bots charge you en masse, funnel them with fences, so that they can only charge you with 5-6 bots at a time.
-Team not listening to the sappers? You are trying to make a fort inside a house by blocking the doors, but the team is leaving? Don't block them out, cuz that is an easy way to get yourself banned, because "you blocked your team"(that was leaving the fort you gave them ample notice about.) Instead, build chevax de frise, and hammer them until it takes only one hammer to complete. Your team can run over them when they are being chased by bots, and then you can just hit it once and bam instant wall.
-Skybases. Careful, this one can get you banned as well, so make sure to give admins/the server exactly what you are doing. The tower section I made earlier? Use the plankjump one. amazing when combined with a ramp, but it can't hold many people, so unless the server doesn't have many people on, don't do it, cuz chances are, you guys will be the last alive, which means that 30 people have to watch 5 people kill 60 bots.

A checker pattern of small chevax seems to work well, so place a small chevax, then place another one next to it and slightly behind it, and then in front, etc. I have not tested it. (comment)

Another thing that I have seen useful against cavalry is multiple rows of fences, placed closely between eachother. the theory is, the cavalry have to try and jump over them, and they get stuck in between the fences. The fences will be close enough so that they can't get enough speed to get over the fences. They would have to turn to the side and try to jump over every fence, one by one, while infantry could shoot at them. It won't "stop" them, persay, however it will slow them down to let the infantry kill them.

Well, I am going to go ahead and state the obvious, forts. The whole point of forts is against the enemy, and the enemy is mostly, if not all, infantry.

Now that that is out of the question, there are some other tactics. Spam is one, and is usually the easiest and quickest way to slow the enemy down. Against players, the goal of a fort is not to "stop" the enemy, because that is mostly impossible. The goal is to slow them down, and give the defending infantry more time to prepare and shoot at the enemy. Most forts are made poorly, infantry charges either overtake the fort or get repelled in 20 seconds. Every second more it takes for the enemy to reach the infantry could be one more infantry on your team reloading their gun and shooting an enemy. One more infantry could turn around and realize that the enemy is attacking on the other side of the base. Every second added could mean defeating the enemy.

One strategy is like a feint. you make a high wall that the enemy would have to jump over or destroy. You leave one spot open. I know, you would think people would be smarter than that, but people try to take the path of lease resistance, just like water. They will usually just assume that you were careless and missed a spot.(which hopefully you didn't, after reading this guide.) You can have an entire second line of defense, or just infantry sitting there. Large numbers of infantry will start to destroy the walls, but a large number will try to push throught the gap. A good skill sappers can use isn't just throwing sandbags onto a ground that has no sand, but decieving the enemy as well.

Another one is specific, and I haven't tested it. Place a dirt mound, dig it all the way up, then place small chevax in it. The comment said that it was hard to destroy(because it was buried in the dirt I am assuming) but it stopped infantry. I would suggest doing this after you have set up cover, because stopping the infantry charge is no good if they just shoot your team.

This is fairly difficult. The main challenges are cannonballs. Grapeshot, aka "cannister", is just a bunch of bullets. Cannonballs? There are only two things that stop cannonballs. Ground, and earthworks(they stop the cannonball, but are destroyed in the process). Obviously, ground is fairly easy, just put a mountain between you and the cannon. However, Chances are the cannon is in the same direction as the enemy, so if you can't see the cannon, you can't see the enemy.

The only solution I have seen so far is two layers of dirt mounds. Obviously, this can be a problem, because it will block your soldiers' field of view, along with possibly providing cover for the enemy. Though it will stop cannonballs, I don't know if it is worth severely hindering your base, however, if you are getting serious artillery fire, it might be a good idea to get done.

This is for those of you who decided to look at this guide to learn how to do basic sappering things, because it has gotten somewhat popular.

Some things to always keep in mind when building things:
-Cav is bad. Bad. REALLY REALLY BAD. Like having a fire on a spaceship bad. Like smoking a cigarette at a gas station bad. Like being in a pool when a grenade goes off next to it bad. Like... well you get my point. Cav and sappers do not mix. If cav is coming towards your base, you better have that base done, or you better have a ton of infantry with you. You cannot run. You cannot hide. The cav will find you, and they will kill you.
-There is no "one-base-fits-all". You need to adapt your base to your surroundings. You are on a hill? Build the base around the hill, you don't need fences or anything, just make sure that your base has ample cover, and is able to shoot down. Your base is in a valley? Well it shouldn't be, thats just plain stupid. Your base is in an open field? You don't need to make a circle base, instead make a main line, if the enemy starts to flank you, build to the sides, but if you can see your enemy, they shouldn't be able to get behind you. You save bp and time by doing this. You will also want to build some defenses against cavalry, such as fences and whatnot, because an open field is a cavalry's playground.
-Artillery, dear god artillery. If you see artillery, or it starts to land near you, make sure you communicate to your team that the artillery needs to die. If the artillery is defended by infantry, well you should have done the same thing. If the artillery isn't, then you need to communicate to the cavalry that you will give your eternal gratitude to the cavalry that brings you the artillery team's heads. Otherwise, The artillery, if they know how to aim and what they are doing, will tear your fort apart, with roundshot. No where will be safe from the roundshot. I have seen artillery get over 10 kills on infantry in a fort, because the artillery had flanked a line fort, and the infantry was just lined up.
-Don't expect infantry to follow you, follow the infantry. Unless there is infantry saying "okay (insert sapper name here) I will follow you and join your fort" a fort is useless if your team is already dead. However, the instant that you have some infantry following you, if you set up a good fort, when your team dies, they will see your fort, and they will join you.
-Don't build a uselessly large or overdone fort. If you need cover, build two sandbags, unless you are in a forest/bushes, then build a small ramp and some sandbags. You might think you have plenty of buildpoints, and you start building this epic base, but as you get to the other end, you start to run out of buildpoints, so you end up with half of a base that is 5 sandbags high, with this crazy ramp system and platforms, and then the other half of the base is just 1 sandbag, and a few gabions. Be efficient.

Recap:
Cav + sappers = death of sapper
adapt bases to your surroundings
artillery + fort = mass death of infantry, plus sapper possibly
work together with infantry
build efficiently, with minimum buildpoints used.

Rockets. They actually aren't bad if you are fighting against them, cuz they couldn't hit your fort from 2 feet away. You gotta be worried when rockets are on your team. They set up behind your fort, and they always end up blowing up your fort. Especially against bots. All the bots will be blobbed up against a bunch of infantry hiding behind some cover, a rocket thinks about how much kills he will get if a rocket hits that, and nothing else, he gets 10 kills, blows up all the cover, bots break through, stab all of the infantry while they are still getting up from the explosion, and then the rocket goes "oops I can't aim it sorry". YOU CAN AIM IT. ITS CALLED HAVING WHAT YOU WANT TO HIT IN YOUR FIELD OF FIRE, AND WHAT YOU DON'T WANT TO HIT OUT OF IT. If a rocket sets up behind your base, throw a tantrum. I am not kidding. No, its not immature, or mean, or stupid. Get your team to yell at them. Get the admin involved. Rockets do NOT set up behind bases, because they will end up blowing up those bases. Only ok places are either on top of bases, in front of bases, or next to bases. If they set up behind your base, that means that they are willing to sacrafice your team's safety and success, so that they can get some extra kills, and not get shot. It is honestly one of the most selfish things someone can do in this game. My own team's rockets are usually what destroys the bases I make, which in turn makes it nearly impossible to win. If there is nothing else you take from this guide, it is that setting up rockets behind a fort should be something that is bannable. Yes, the rockets can get a lot of kills, yes, they are exposed without cover, yes, its pretty much near suicide to sit in front of or next to bases. However, just the fact that they are even willing to risk, no matter how small the risk is, of blowing up their own team's fort, is wrong. At least with trolls, they are trying to have fun, and they just do things to annoy you. The fact that many rockets are willing to kill their own teammates, sure not directly, but by destroying their cover so they can be cut down by the other team, just to get extra kills for themselves, should say something about who they are. I am sorry to those few rockets out there who actually include the safety of their teammates in their rocket-placement decision process, but most rockets are bad news.

Don't do it to extremes. You can mess around and have a laugh with sappers, but make sure that it isn't at the expense of others. Some examples:

Dos-
Build weird sculptures using random junk

Make a ugly looking fort

Make a musician stage

Build a tree

Build a house

Do sapper charges

Build funny shaped forts

start the napoleonic space program with a mound of dirt

Build something for a flag bearer to place his flag on

Make a store/business/resturaunt

Become a castaway on an island

Build a church worshipping a small chevax de frise, or some other strange thing

Build a church to worship admins(usually safest option in the game for non-utilitarian builds)

Dont's-
Spam TNT in a house

Spam junk at house entrances to stop enemy infantry from breaking in

Blowing up your own team's fort

Blocking your own team

Using dirt mounds to block entrances into houses

glitch into roofs of buildings

build a fort that enemy infantry can't get to, and sit in it alone while outnumbered 20-1

Build a fort in a useless spot(building a fort on an island that you are the only person on, while your team is on the other side of the map)

waste BP(sometimes not even intentionally, if you are new to being a sapper, don't start building your own forts, help another sapper, learn from others.)

Abandon your team(if your team is willing to sit out in the open and get shot at while you are building cover for them, you should be willing to do the same. Don't run away because some bullets are getting near. A sapper is really only as good as the amount of infantry he has with him. If he has no infantry, he isn't useful.)

Build unreachable flying fortresses

leave a finished fort without being under duress(in other words, don't build a fort, and then walk away from it. Tear it down for the extra BP, unless you are being attacked.

To sum it all up, if your teammates are laughing, then that means that it is probably a good idea. If your teammates are telling you too stop, then you should probably listen to them. Sappers rely on teamwork and coordination, the more infantry like us, the more likely they are to follow us and protect us. Stay on your teammates, and the server admins', good side, and you should be fine.

Treat buildpoints like Mr. Krabs treats money. Sappers become a whole lot less useful when they are out of buildpoints. Make sure to tear down forts that you leave. If your team is moving to somewhere else, try to tear down the fort you are leaving, if its going to be abandoned. Even if you have thousands of buildpoints, its always best to get rid of forts, because so many times before a group of sneaky enemy infantry will take over the fort as soon as you leave. That can be a major problem, for obvious reasons. But lets get back to buildpoints. The whole idea of sappers is to build cover. Pretty much your sole purpose in the game is to stop bullets from performing unexpected brain surgery on your teammates. You build things, you dig things, you break things, you float things, you do things, with the sole intention of making your team less killable, and the enemy team more killable. Without buildpoints, you suddenly go from the great defender of musicians, protector from bullets, savior from cavalry to a guy with an axe that maybe, MAYBE, if you are lucky, can one hit an enemy if you hit them square in the head, without being stabbed while you are slowly trying to swing. In other words, without buildpoints, your resume pretty much drops to a few sentences, "can absorb 1-2 bullets.", "can use axe with very poor ability to maybe kill one really, really terrible or unlucky enemy", and "after teammates die, can try to pick up one of their weapons and fight on." If you take anything from this guide, watch your buildpoints. It always seems like enough buildpoints until it isn't, and then you have to scour the map for anything your team built and destroy it, all while trying to avoid enemy cavalry, infantry, and the other-ry.

If you are going to use tnt, then try to hide it. A good trick is to hide tnt at a door, then place a line of tnt away from the door and to some hiding place. You must hide all of the tnt, then wait for enemy infantry to enter the room. Wait for them to walk a ways past the door, then detonate it. The idea is that the infantry will panic, and try to run away through the door. You have to blow him up before he reaches that door. Hiding the tnt is nearly impossible though, because the instant the infantry sees tnt, they are going to either run, try to detonate said tnt, or both. You also can't cover up the tnt with sapper stuff, cuz a big pile of random sandbags, planks, gabions, chevax, and whatever else you can stuff into a pile doesn't look suspicious at all. In general, tnt is just not really that effective. The only use I have seen for it is if the enemy barricades a passageway, the tnt will blow up all of the fortifications. Against forts, its pretty difficult, because you need to be able to actually stand still next to the tnt, which will be next to the enemy fort. If the enemy doesn't shoot you, then you are dreaming, pinch yourself in the face and wake up.

Tnt is actually useful against bots, because it is bulletproof. This usually gets you banned from servers with bots, but build a box around you made of tnt, at least 3 tnt high (to stop sudden lead poisoning), then wait. Bots will gather around your box, and then you detonate it. You probably will not survive this, and you usually only get ~10 bots, so if 10 kills isn't worth possibly being kicked, and at the very least having to wait to respawn, then don't do it.

A summary: tnt is only useful against a really stupid infantry in an extremely lucky location with massive amounts of planning, or it is good at destroying barricades that the enemy is not, I repeat NOT using as cover. It also is good against bots, but runs the risk of being banned.

This guide is not finished, but overall, I am suprised so many people are checking this guide out. I mean, its among the 10 most popular guides right now. Thank you for that. Feel free to leave a comment about anything I misspelled, or even other sapper builds, or tricks. I will read through them, and if its good, I will add it. Of course, if I don't add it, I might not have seen it, but if its something like "chevax de frise around the base" or something, I won't add it, just cuz, come on, if someone isn't able to realize that building stuff around the base blocks cavalry, they shouldn't be a sapper. Overall though, I have pretty much finished most of the guide, this will be a continuous work in progress, so you can check back and see the new stuff added, but I have added most of the sections I think will be needed. Thanks for reading the guide, hope you guys liked it.