In the deep, there are beasts so fell and terrible, that only they know what they are, for none who have met them have lived to tell of it... they are the Forgotten Beasts, born of the chaos from before the world's birth... they have waited, brooding in the dark places of the world... and now... by digging too deep... we have awakened them. - DF wikipedia
Forgotten beasts are the subterranean equivalent of titans; large, monstrous, procedurally generated megabeasts that spawn in the caverns or HFS beneath your fortress, pathing through to your fort and seeding destruction amongst your dwarves. They are attracted by wealth and population size. Additionally, one may randomly appear when you reclaim a fortress. When a forgotten beast becomes visible onscreen, the game automatically pauses and displays a message. - DF wikipedia and HFS = Hidden Fun Stuff

Types of attacks :

• None
  
• Noxious Secretions
  
• Hunger for warm blood
  
• Poisonous bite / sting
  
• Spitting
  
• Fire breath
  
• Toxic blood / vapour / gas
  
• Deadly dust
  
• and lastly " webs

Material Composition :

Body Structure:

The procedure is to simply have a tapping into the caverns, then once you know where they are it becomes much more simpler to work on it.

We do the tapping to later make a connector between all three caverns so knowing the location is important.A normally experienced player will go the long way and just dig straight down from the main fortress but if you actually decide to plan it all out for the long run then this is how you tap in. There are always consequences but yes, risk is a risk and saves the effort of tapping in.

The tapping is shown as follows:
Steps:

• Make a small Hut like structure 3x3 walls with floors on top and a door/bridge to enter inside it.
  
• Now straight dig down. When you encounter the first cavern, block it off, dig into the pillars and go down.
  
  
• After 2nd Cavern, dig again straight down.
  
• Now you will encounter the third cavern. We stop here.
  
• While encountering and digging deeper, make sure to have walls, doors and raised bridges as a fail-safe so that work may proceed.
  
• There's lot of stuff down there that can cause problems but a few doors / bridges and walls do the trick. Either ways the top " Hut structure " is a fail-safe.

Now here we can see how the tapping totally isolates the fortress and at the same time we find the location of the Caverns. Of-course you can use the Sonar but this is far much more sophisticated and non-cheat way of finding the caverns. Safe even at times, unless a flying forgotten beast is in there or cavern dwellers, but since we are isolated from the main fortress we can sacrifice that one unlucky miner over the safety of the fortress. That's how it is sometimes.

Why we make a cavern connector is because we don't want to make matters complicated. Why have multiple routes to the fortress when you can just have one straight route to the fortress!?
So what we have done here is, after knowing the locations of the caverns we plan out a route to the most easily accessible region of the caverns and then just make a door/bridge and then dig directly to the cavern layer.
Making a door or a bridge ensures a fail-safe mechanism just in case there is a lot of invisible cave-folk down there in huge numbers, you can lock the doors in or raise the bridge, thereby reducing the casualties to minimum.
Now like as shown in the diagram we have a straight downward pathway of stairs and this stairway has connections to all the caverns. By doing so we have made the tasks for forgotten beasts to come to our fort very very easy.
They will just walk and enter these doors/bridges and come to the fortress.



I really hope the image and the details mentioned are self explanatory. Just connect the three caverns with doors and bridges. But, while doing so, we have a trap room. Now I will explain it in the next section.

Now, we have found the caverns, connected them and have the caverns closed for time being. There is a need for a proper trap room to be set up. The architecture the older players set up is gigantic yet highly effective.

Now there's two things to keep in mind when doing such high level dwarven engineering, the risk and consequences.
You can,
Go the smaller approach and use much less safety and risk it. (I myself follow this)
Go with a lot of protection and isolation methods.
Go with the larger and distributed approach so that area wise isolation is much easier.

Now there is the large, small and very small route based layouts, giant 3 cavern trap rooms connecting various sections for those 4x4 or 5x5 type of maps with giant sectors connecting the trap. It is not like you need to make them big, go as per your need, one or two time delay are more then enough.

Now I will show the trap room and the layouts. I tried fiddling with it and many things are more or less. It isn't really perfect without the raised bridge concept. Raised bridges used to be a full-proof and safe method over doors. Doors make the job of isolation quite easier since the premium version has come out. But one day it won't be and when that happens, it is an invitation to tremendous amounts of fun.
Let us move on to the next section.

This is again going to be written separately as a Silk Farm Guide
There are three known ways to create webs.
You use a Giant Cave Spider.
You use a web spitting forgotten beast itself.
You use small vermin cave spiders. ( I do not know anything about this myself)

Giant Cave Spider Silk farm and using it to catching forgotten beasts is the same mechanism but with a much compact layout. Here is the layout diagram and explanation is below it:


Now I will just tell in brief how to do this,

• Catch a spider and place in a small room.
  
• Make the room as shown in Diagram or the last picture in this section.
  
• Be sure to add draw bridges.
  
• Be sure to add fortifications and doors.
  
• Be sure to link all bridges to levers.
  
• Be sure to make a retractable bridge with a lever attached for the bait/trigger drop from 1 z level high.
  
• Be sure to have cage traps for Giant Cave Spider based trap room. If it is a forgotten beast, once it is trapped, you are done. The best way if you wanted to kill it is using retractable iron spike trap at the Spot where it will be trapped in.
  
• Now once the bait is in place, just lower the bridge near the cage traps and the web spit will go all over the cages. The cages and spit webs better be forbidden from the dwarves. Don't want no dwarf getting caged in the cage. Yes, the dwarf will be trapped in the cage and can be traded off and can also be released by placing the cage and then releasing it by pulling the lever.

And this would be a better in-game picture for the farm.


Layout Types:
One Side Compact
Two Sided Standard Design
There is only on difference in 1-Side and 2-Side design, that is the efficiency and flexibility. The one sided is for like the compact one.

Actual Design:

This is how the trap room looks like.

• A narrow pathway.
  
• A line of cage traps everwhere.
  
• A web-spitter.
  
• A trigger for the spitter to spit webs on the cage traps.
  
• A mechanism to stop the trap.
  
• A mechanism to load the trigger of the spitter.
  
• A mechanism to Safely go in and perform maintenance.

This is an image of a dual, small and manually operated farm from a very professional player.
Image has been taken from reddit. It was posted by : u/SoulfulZane

Now we have the rooms, trap room, web spitter, bait, layout and all. Now if you have the webs spit out on those cage traps, the FB will get trapped in the cage, in a guide some day, I will make a very beautiful guide on how to please Armok by making extremely FUN traps using FB's. But today the guide here can be said as partially completed.

Now the hardest way to do this entire process is to use a Forgotten beast. Why? because of getting it into the right spot is very hard. But there are ways to make it a little easy. You can lure it at a spot using a tied up dog or a cat. You can even use a goblin or an elf that got caught and you stripped it and tied it at the spitter spot. The trick is to slow down time and to lock doors and use draw-bridges at a very important time.

I will add a picture to make this data much simpler to understand. I understand that this guide is like a very complicated and high level stuff but trust me, once you do it, it is very easy. Also, you can always save the game and repeat till you succeed once in catching the beast that spits webs. If it is a cave spider, it is really very easy at that point, just don't tame it else it is useless.

These would be the last examples of trapping in a Forgotten Beastie:

Trap in action :

https://youtu.be/QdVsybE6yag

Lastly, Good luck. I hope you get a cave spider over a forgotten beast. long passages with bridges and doors will do the trick. Currently doors are very very over-powered so you can go ahead and forget about bridges but when they won't be OP, then problem will start. The web spitters must all be killed or just plain let in long corridors and just locked up. You can lead them into hell or pour magma on it or something like that. Entirely your wish. I prefer baiting them into a room full of iron spikes that go up down repeatedly. Does the job... yes that would be all. Thank you for reading. I hope I helped. Guide will be updated as per suggestions and more data.

From the original guide, many things are to be added, I will just publish this for now.
The main links to this guide are from here :

https://docs.google.com/document/d/12LEc-oCLmDHeTLXjetZ-qy5FoaI50hXekKWX4zdSm8E/edit

http://www.bay12forums.com/smf/index.php?topic=180198.0

Guy trapped 743 beasts... in ASCII... hehe.