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[Quarque]

So I've been trying some trap designs to kill forgotten beasts, in a fortress with access to magma and water.

The plan was to lock them up in a hallway with bridges, flood it with magma and then add water. Kill any type of fb safely, leaving no poisonous excretions, poisonous blood or whatever. It works, but I'm looking to improve the design. What I have right now, sideways view:


__W__________W__    <- water chamber, floor is a retracting steel bridge, can be filled to 4/7 from reservoir
__W__________W__    <- magma chamber, floor is a retracting steel bridge, can be filled to 4/7 from reservoir
__W----------W__    <- floor covered with steel floor grates
__BD________DB__    <- killing zone: steel doors surrounded by raising bridges

As soon as a forgotten beast turns up on any of the three cavern layers, I open another raising bridge that opens a path from its cavern layer into the fortress, leading through the killing zone. (It first leads through a hallway with engravings of all the forgotten beasts that died before in the fortress). As soon as the FB is in between the two steel doors, I lock both of them. Then I order the two raising bridges to the left and right of the doors to be closed, dump the magma, wait a moment to enjoy the sight of a burning FB if it is fleshy, then dump the water. They can't destroy the grates from below, because they can't path there.
They should be able to destroy the doors, but that takes long enough to have the bridges close first. Actually, for some reason no FB seems to be able to destroy the doors at all - I think this might be a bug in the most recent version of DF.

However, the problem is that cleaning up afterwards is time consuming. Digging out the obsidian from the killing zone is fast enough, but on the level above, the grates are choked, because the obsidian casting forms a floor there.
So I need to deconstruct the grates, channel out the floor and then rebuild the grates before the trap can be reused. A cumbersome step that I would like to skip.

Is there a way to make the trap more efficient? I have been thinking of making the hallway of death 2 z-levels high, but then a flying fb might survive? 

[Fleeting Frames]

Next time you remove the grates, dig downstairs instead of channeling - they'll remain behind after obsidian-casting so it'll be last time you need to deconstruct the grates.

Personally when building from scratch I'd also try to cut down on z-levels, but like you said potential flying FB might survive.

[Quarque]

Brilliant, thanks!

[PatrikLundell]

The FB behavior did indeed change in the latest release, and I've yet to see one trying to destroy a door: they just path back and forth in between them. Meanwhile, trolls, ogres, and some experiments have no trouble destroying doors. I've checked the FB flags, and they indicate they should be building destroyers, so I assume the AI has been changed to ignore doors to try to get through instead, but didn't enable destruction when they're locked up.

[Leonidas]

I've also found FB behavior difficult to predict. My old standbys of statues and chained kittens don't work consistently. Plus, FBs in 47.04 move faster than I remember. Are the FBs consistently attacking your steel doors from across the map, or are you setting out other bait?

[Bumber]

Next time you remove the grates, dig downstairs instead of channeling - they'll remain behind after obsidian-casting so it'll be last time you need to deconstruct the grates.

Does this work for bridges too?

[Quarque]

Are the FBs consistently attacking your steel doors from across the map, or are you setting out other bait?

I open up a pathway into the fortress as soon as a fb appears on the map. In most cases, it will attempt to walk in when possible.
If it doesn't, I use a squad of one dwarf as bait with a station order. As soon as any military is within a (pretty large) radius of the fb, it will chase. No matter how long the pathway is.
Then I order the soldier to flee back into the civilian burrows. So far, no soldier was caught.

[Moeteru]

As soon as any military is within a (pretty large) radius of the fb, it will chase. No matter how long the pathway is.

That's a very interesting finding. You could probably make it completely safe by digging a tunnel near to the FB without actually breaching the cavern. Your dwarves could be close to it while keeping your whole trap system between it and them.

[Schmaven]

What about gambling on no magma immune flyers with something like this:
______________
__B_______B____DB_   <- water chamber, floor is a retracting steel bridge, filled via drawbridge from one side, and another killing zone on the other
__BD______DB__       <- killing zone: steel doors surrounded by raising bridges (same footprint as the water chamber)

Killing zone top view:
__WWWWWWWWWW__
__BD______DB__       <- killing zone: Bridge opens at side and larger magma chamber quickly fills killing zone with magma
__WWBBBBBBWWWW
__W________B__       <- magma chamber, filled from magma source via drawbridge
__W________WWW
__WWWWWWWWWW__

I'd bet that the water would fall and obsidianize them faster than they could fly upward out of the magma and into the water chamber.  If they do make it that far however, you could just have another killing zone style room connected to the water chamber and just keep them in there for eternity or come up with another way to kill them.

It seems like a fairly uncommon perfect storm of FB traits to get one that is both flying and magma immune, but maybe I've just been lucky in my forts so far.

[Fleeting Frames]

@Bumber: Yeah, it does iirc.

(Though if using roads/farm plots those may not work out as desired yeah.)

[muldrake]

It seems like a fairly uncommon perfect storm of FB traits to get one that is both flying and magma immune, but maybe I've just been lucky in my forts so far.

I have had it happen once.  I cleverly poured magma on it through a grate but because I had missed a path to the surface, the thing could actually destroy it from below.  I don't think it was completely immune but it also was not particularly injured by it so it had enough in it to deliver some fort-ending !!FUN!!

[anewaname]

That perfect storm harbringer of fire and smoke can happen.

[knutor]

Been taming snakes in my fort. 1 rattlesnake just killed a nightcreature. Surprised me, it did. Rattlesnake got its head ripped off by the werebear nightcreature, however, it did get the last laugh. Im guessing its bite, paralized the werebear, because it never swam out of river they were fighting in. Werebear deathcause (drown). The right combo of damage should do it. Are you after FBs bodyparts, because Magma traps maynot leave any. Especially if its hardened into obsidian. Maybe just use a bunch of magma crabs and fire, like a room with coal block flooring?

[PatrikLundell]

Floors do not burn in DF. Paved road can burn, however.

FBs are immune to most everything, including stun and paralysis.

An obsidian trap implemented by water first and magma second results in a recoverable corpse and items, once you dig out the obsidian. Magma first and water second may obviously harm everything that isn't magma safe.

[Fleeting Frames]

Water first is vulnerable to FBs made of fire, though, so I prefer the other way around (that goes still relatively quickly).