In the current version 31.25, my archers gets bugged when I get more than 50 dwarfs. It is a problem when a poisonous Forgotten Beast comes in.
Sinse FB's are immune to traps, can they trigger pressure plates to activate a ballista or fluid traps?
FB's are destroyers, so I'm thinking about placing a wooden door or floodgate to unleash some fluid upon destruction, so the fluid can trigger the plate. Can I activate a ballista with a given pressure plate?
Two things. FB's are immune to automatically triggered traps. They are not immune to other types of activated traps, or the Results of reactions generated because of the actions of traps.
1) Magma + Water = Obsidian and everything is dead - except ghosts. There's a link to a chessboard obsidian mixer on the forums somewhere which is a pretty much foolproof way to kill anything, including clowns.
2) Spike traps can be linked to pressure plates set to activate at a certain level of water. Set up a room of size 14 squares with a door and a 1x1 alcove with another door behind it. Then set a bunch of water pressure plates set to activate at 7, then fill the room entirely with water, so it'a all at 7, and release one alcove room worth of water so you have a perpetually swirling depth 6/7 room of water (can't remember what happens to the water in the tile with the floodgate to the alcove - might not even need the alcove. Could also be done with a 1x1 watersmasher bridge. Then lock it all up and never touch it again, except when you block off other entries. Mark the entire area forbidden to dwarves,use burrows, whatever but let the FB's be the ones exposed to FB blood and ichor, not your dwarves. As time goes on, if you get artifact doors or floodgates you can use to control access to the killing room, use them.
3) Just dropping ceilings on things is pretty satisfying too. A room with a hanging ceiling, supported by a single support, there are many different ways to handle triggering such a collapse.
4) Non-flying things can simply be dropped about 10 Z-levels and splatter. If they survive, well, they can be moat pets.
5) Look up the Goblin Grinder defense system. Very, very nice, simple, and elegant for most standard attackers.
If you want a completely military-free defense system, you want to stagger your defenses, from commonplace to extreme. I do them in this order - and it takes years to finish them in most of my forts because the defense system tends to become a megaproject.
1) Cage traps
2) Stonefall traps
3) Goblin-grinder type weapon trap systems
4) long-fall dodge traps, or waterpressure toss trap
(3 & 4 might actually be combined into 1 if you are creative)
5) Then you get into the things to stop flyers / building destroyers, etc.
6) Autocycling spiketraps that your dwarves are completely forbidden from entering without a lot of effort on your part
7) Chessboard magma mixer.
The cage traps allow you to catch animals for foor / taming / breeding / fighting practice / whatever before they hit serious defenses. Stonefalls beat things bloody and will kill them without creating too many body parts to pick up later. Goblin grinders, if a large force of anything hits them, tend to make LOTS of jobs for your dwarves during cleanup.
You may want to set up a bypass around the mundane traps for FB's, to keep them from gooing up the place. This would need to be manually activated.
Anything designed to handle FB's or building destroyers needs to be upgraded to artifact (indestructable) items ASAP. Doors and structures should be replaced with walls as needed.
Any flow sources need to be designed so that the floodgates are next to fortifications. It IS possible to build a wall, THEN turn it into a fortification, so you can build your cisterns, set everything up, then build fortifications in front of the floodgates so no building destroyers can ever touch them. For my trap systems I typically build completely sealed water and magma sources with no possible dwarf entry point unless I deconstruct fortifications. Floodgates are controlled by pressure plates so I don't have to worry about stupid angry dwarf (or ghost) lever tricks. An emergency lever system might be kept in a walled up room with no possible entrance if it's important enough.
Freezing water traps in artic extremes are possible as well, but some ice environment critters can survive freezing I think. I don't much care for artic biomes.
There are so many ways to do awful things to your enemies in DF