Imagine this.

You and your team have been pushed back almost all the way to the hatch. After a hard and bitter fight, you've managed to eliminate all the immediate threats to your victory, and you're now ready to push back up and take back that ground you lost. And push back up you have (holy hell, what a comeback!) You smile to yourself as you rack up damage with your sentry against those big, lumbering giants, and as the last subwave is about to spawn, victory seems almost certain...

Then you hear it. That dreaded bomb alert, appropriately accompanied by an orange flashing of your bomb compass. You all panic and drop everything in a mad rush back to the hatch. The Heavy killbinds and clicks like hell in an effort to get to the deathcam and buy back in as fast as possible*. The Soldier and Demoman** launch themselves into the air. But it's all for nothing. You're all too late. The sneaky bomb carrier scores. You've lost.

At this point, this hypothetical situation can go several ways, but one thing is clear: you've taken a risk, and your team has been punished for it. That hard fight they fought at the hatch? The struggle following that to take back as much territory as possible? The mad dash to the hatch when they heard the alert? All wasted - all because you didn't guard the bomb. It doesn't feel good, does it?

All right, that's enough hypothesizing from me. Let's get to the point of the guide.

Admit it: you've probably heard someone at some point say that the Engineer should keep the sentry on the bomb when it's behind the frontlines - a situation where, after the bomb has traveled far enough, the robots could go every which way to get to it. (You may even have been the Engi who screwed up in such a situation in the past.) It's not worth making the whole team stay back just to kill those few little scrappies who sidestep the main bomb path, and your team needs to keep as much ground as they can in case things go wrong again... but you can't just leave the bomb there, unguarded, and pray that the wave ends before that one stupid little bot scores the bomb. You've got to keep some source of damage to guard the bomb, right?

"[Placing the sentry directly over the bomb]'s funny sometimes." Screenshot courtesy of itsgalf.

So why the sentry? And why "on" the bomb? Has anyone actually tried to explain those to you? No? Well, maybe this guide will help with that.

*I remember doing this a few times in the past as Heavy. I'm not saying this is a legitimate strategy, but as the slowest class in the game, if you think you'll have to run back and kill anything that gets past your defenses and too close to the hatch, this is probably your best bet.
**Assuming you have both of those classes, of course. You can have other classes to "replace" them as long as your team's weak points are covered.

1. The sentry doesn't miss.
   It's easier to trust something with legal auto-aim and autofire to guard the bomb against small, singular, fast targets than it would be to trust an unskilled Sniper, a lagging Soldier/Demoman, a Scout who can't coordinate his movement and shooting very well (hi), or even a Scout who blew all his money on resistances and can't do damage for $h1t, to give some examples.
   
   
2. It allows everyone to stay in the frontlines.
   Because everyone still has jobs to do. Your DPS classes need to keep up the DPS against the robot horde. Your Scout/Money Spy needs to make sure most (not necessarily all) of the money is collected. Your Medic - assuming you have one - needs to keep the team alive. And you need to stay with your teammates to keep your dispenser up and near them so they don't have to run around looking for ammo.
   
   
3. Your damage classes (usually) can do more damage than the sentry.
   Okay, maybe this isn't always the case. The sentry's DPS is actually quite good in the early waves, and it's not uncommon to see the Engi lead in damage after the first wave. However, the sentry can't dodge, has high aggro priority, attracts Sentry Busters and Spies, is squishy in the early waves, and its DPS can't be upgraded as much as that of most other classes. Do I have to give any other reasons why the sentry shouldn't be one of the main damage dealers?
   
   Sending one of your damage classes back to guard the bomb will more than likely leave the team with insufficient damage output at the frontlines. On the other hand, leaving your sentry on guard duty will still allow the retention of most of the team's damage potential at the frontlines while still keeping a good enough damage source against most things that manage to get through your defenses.
   
   
4. It makes your job as Engi easier, period.
   Believe it or not, playing the safe way with your sentry and the bomb is often the best way, for several reasons:
   
   
   • If your sentry and the bomb are very far behind your team, you won't have to worry about attracting Sentry Busters so often, and even then it won't do much harm to let the Buster blow up your sentry so you can insta-build a new one and get back to the frontlines with your team.
     
     
   • Even if your sentry and the bomb aren't that far behind your team, you'll still have a much easier job dealing with Sentry Busters and Spies than if you were to put your sentry right in the frontlines where it would probably take fire from most of the horde on top of still attracting Busters and Spies.
     
     
   • If your sentry and the bomb are far enough away, you won't have to worry about your sentry shooting at you on maps like Casino City (a custom map; look it up) and the forwardmost dispenser spot on Rottenburg when you walk up to your dispenser to move or repair it.

Overall, the sentry is a lot better served as the team's safety net than one of the main damage dealers when the bomb needs to be kept under watch, and I'd say it does the best job of keeping the bomb safe from stragglers as long as the threat isn't too powerful (looking at you, Super Scouts).

This question is arguably a lot less important than the first question, but I think it still deserves some weight.

1. It gives the sentry an easy shot against stragglers.
   After all, it's much easier for a sentry to aim at something running right at it than something passing by it at a sharp angle. And when the sentry has an easy shot, it won't spend so much time turning to aim at the target without shooting.
   
   
2. It gives the sentry a good field of view against robots coming from any direction.
   Pretty obvious one. You don't want to put a sentry three meters to the right of the bomb just to have robots coming from the left get extra time being out of the sentry's range to inch the bomb forward and trigger the alert (if it's really that close to the hatch) over and over.
   
   
3. It makes it harder for incoming robots to actually pick up the bomb.
   Again, pretty obvious. Maybe not that much harder (depending on how accurate you are with the placement of your sentry over the bomb), but the difference will make itself felt. Or it won't. And it's just funny sometimes. There, I said it.
   
   
4. It's an easy instruction to give and understand.
   All you really have to type is "put/keep sentry on bomb" or "sentry stays on bomb" and the target Engineer will most likely know what you want them to do. You could phrase it differently, e.g. "guard bomb with sentry", but that might get them confused over just how they should be doing that. I think this is also the case for when there's a language barrier between you and the Engi, but I'm not too sure.

Overall, I'd say putting the sentry on the bomb is the most reliable place for it when you need to guard it, and it makes for an easy way both for Engis to understand how to guard the bomb and for others to tell Engis how to do it. Just try not to get too OCD about putting the sentry right on the bomb and leaving no part of it uncovered, and all that.

1. When the bomb is NOT behind your teammates.
   Maybe the wave just started. Maybe the bomb's been reset. Maybe some giant/boss has been carrying it for quite some distance and won't die. Maybe your teammates are good enough that the bomb doesn't slip by at all. Maybe some bozo's trying to stall the bomb indefinitely with whatever knockback-laden crutch they're using. Whatever the case, mindlessly keeping your sentry on the bomb in the likes of such situations will more than likely get you and your sentry killed quickly and make you look stupid.
   
   
2. When you and your team are controlling a single chokepoint that robots MUST pass through.
   This one's more up to your own judgment and map knowledge. If you're familiar enough with the map to know that robots cannot bypass said choke, it's much better to move your sentry up to there so you can contribute more overall DPS for your team and more quickly kill anything that escapes your team's defenses. It's also helpful for your money collector, who won't have to run so far back just to get the money from whatever your sentry kills.
   
   
3. When the threat(s) in question are giant Scouts.
   Giant Scouts have much higher HP than most other threats and so are more likely to power through your sentry's defense to steal the bomb and run past it before dying. However, they like to take the tightest turns possible to get to the hatch, and you can exploit this by putting your sentry next to a tight turn/corner that you know the incoming Super Scout(s) will take to get to the bomb, whether it's in front, behind, or still very close to the bomb.
   
   
4. When there are multiple bombs.
   I don't think I need to say much about this. If there's more than one bomb, just try to put your sentry in a spot where it'll guard as many as possible. You might have to let them advance just a bit.

Overall, it's a good rule of thumb, but don't just follow it dogmatically. Play more games and try to rely more on your own judgment when you follow or break this guideline.

Sentry?: Doesn't miss, makes your job way easier, & lets everyone stay up front to keep the most damage potential there. Overall best guard of bomb and better as safety net than a main damage dealer.

"On" bomb?: Sentry can shoot stray robots easily, has good FOV vs. bots coming from any direction, & makes picking up bomb harder; also an easy thing to both say & understand. Just don't try to micromanage putting it right on the bomb.

Exceptions: Bomb is in front of team / has been reset, or you control a choke in front of bomb which robots MUST pass through, or you need to sentryblock giant Scouts running for bomb / running for hatch with bomb, or many bombs need guarding.

12 Jan '20:

• Fixed some minor grammar mistakes and missing info
  
• Added "Further Reading" section

14 Jan '20:

• Added Underscore Gaming's newest MvM video guide to the "Further Reading" section.

1 Mar '20:

• Added another statement in the "single chokepoint" point in Exceptions: keeping the sentry in a choke closer to the frontlines (that you know from experience the robots MUST bypass to get to the bomb) has the added benefit of making your money collector's job easier, as they won't have to travel so far back and forth just to collect money from your sentry's kills. Thanks to (っ● ‿ ●)っ for pointing this out.

If you want to really up your game in MvM outside of simply sinking hours upon hours practicing it, here's some great guides for you.

itsgalf's Engineer guide, which also links to several other good guides on the class: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=547951496

What you should avoid upgrading: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=732848812

An absolutely stellar one that basically started my journey of reading good MvM guides. It explores the gamemode more in-depth, touches on some Mann Up-specifics and links a lot of other good guides: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=500673166

A good guide for the more veteran and the somewhat lost players that debunks most (if not all) the misconceptions that you might have "learned" from Two Cities: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=289059169

Scoutfingers's guide collection: https://gtm.steamproxy.vip/id/Scoutfingers/myworkshopfiles/?section=guides

Everything that works in MvM, including recommended upgrade paths: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=461752155

NEW: A recently published video guide for the Spy (which very few people know how to play properly) made by Underscore Gaming, formerly known as CAR_. I've been a fan of his guides for years, and this guide was just so well made that I can't let you guys not know about it. https://www.youtube.com/watch?v=8uLTixBXTPM

Thanks for reading this wall of text. If there's anything I missed, just let me know, and I'll take note of it and try to fix it.

Now get back out there and fight the robot menace. And try to make sure you don't lose because a bomb wasn't guarded properly.