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[Derakon]

I know designs like this have been discussed in the past, but this is the first time I've put one to actual use, and it seems to be working well, so I thought I'd share the design with some screenshots. The goal here is to make a trap that kills your enemies with magma, without having a long cycle time while you wait for the magma to evaporate away, and with letting you retrieve your enemies' equipment as conveniently as possible. We do this by having a three-layer trap: layer 1 pours magma onto bridges; layer 2 is floored with grates (catching equipment but letting fluids through); layer 3 holds the drained magma and lets it cool off well out of the way.

Here's the trap, at the bottom of the screen, along with my farms, glassworks, and food storage. What we have here is basically a long, twisty tunnel floored with retracting bridges (over open space) and walled by fortifications. The upper-left bauxite bridge is holding back magma pumped up from the magma pipe (you can see my axles carrying power from an array of water wheels over the brook). The upper-right bauxite bridge is failsafe #1 for preventing enemies from reaching the interior of the fortress. The top-right green glass bridge is failsafe #2. And the water pumps on the right are failsafe #3; I can flood the entire area if I need to. They were originally meant to clear out the tunnels, but then I discovered that even 1/7 of magma will make an obsidian wall when solidified.

And yes, the butcher's and tanner's are surrounded by green glass walls. I don't know.


One level lower down. This is my loot extraction facility. After burning my enemies with the magma above, I retract the bridges, dropping burned enemies and their equipment down to this level. Here, the magma drains through the grates, leaving equipment behind. In the off-chance that an enemy avoids the magma, I can wall off the entrance to the area using the bridge in the upper-right.


One level lower down again. This is where the magma drains to. The four glass walls are supporting floors on the level above (since grates don't provide support). I can dig this level out more if I need more room for magma to evaporate.


Orcs are attacking! Oh no! My fortress of fifteen dwarves is surely doomed! Failsafe bridge #1 goes up, barring entry...


And the magma is unleashed. The magma release lever also controls floodgates to stop the orcs from escaping or the magma from blocking up the corridor.


Closed off the magma and retracted the floor bridges, dropping everything down. I did that in the wrong order, so some magma slipped past the now-open floodgates, blocking things up. Oops.


The loot extraction facility. You can see some orcs (and their beak dogs) survived. That's OK. I'll terrorize them with random drips of magma from the upper levels. The access bridge is a few frames from being closed.


And finally, the magma evaporation facilities.


If I were to do this again, I'd change the following:
 * Disconnect the glassworks magma supply from the trap magma supply. My glassworks deactivate when the trap is running because all their magma gets stolen.
 * Use something other than fortifications on the top level. The problem is that sometimes gear gets tossed into the fortifications instead of getting dropped down a level, which makes it annoying to retrieve. It's a minor loss, though; mainly just looks ugly.
 * Move the entire trap a bit further from my fortress. I can't drop the entire twisting corridor down to the level below because an aqueduct is in the way. Thus some of those bridges are actually drawbridges, and exist solely to crush magma.
 * Put the water failsafe and the magma pump on different axle systems. I had a flooding issue because I missed a pressure leak, which gummed up the magma works. It's fixed now (the random yellow pump in the second image is a fluid-blocking power transfer, for example), but it was annoying to clear out all the leaked water (see also: water-crushing bridges in image #3).
 * Add another level between the grates and the magma evaporation, so I can turn magma evaporation into an obsidian farm.

[AlienChickenPie]

A loot collection level separate from the slaughter level sounds unnecessary. You could just have a level with a magma pool and a bridge of grates above it. Enemies walk into grates and an array of pumps at the same level as the grates causes the magma level to rise above the grates and burn any creatures on them. The pumps are stopped, and then a floodgate partially drains the pool to bring the magma level below the grates. The drained magma is reprocessed or disposed of.

[Derakon]

The issue there is getting adequate flooding on the kill level. You'd need a massive array of pumps to get any decent kind of coverage, since the magma would be continually falling back down into the pool. In fact, I'm fairly certain that if you  pump onto grates, you'll have pretty low odds of burning anything at all -- the pump activates and moves magma into the square in front of it, but that magma then immediately drops down again. Or am I misunderstanding your design?

Your design would also require my dwarves to go to the killing area to retrieve loot, rather than the (much more conveniently-located) loot extraction area.

[Randominality]

i did something very similar to this in a fortress http://mkv25.net/dfma/map-5258-dwarfheavenwithgoblincinerator a while ago. Except its 4 levels not 3.
1. The goblins walk over my entrance bridge, the lever gets pulled and they fall down to the next level.
2. They fall into a chamber already filled with magma with a retracting bridge as a floor - they get incinerated and the floor retracts so the armor falls down to the next level.
3. The armor lands on the grates next to my forges ready for collection and the magma drains down to the next level.
4. The magma now is channeled into a room above which is a water dumping bridge to obsidianise it so it can be dug out and used for statues.

It works pretty well but there is less time to drop the goblins than in your setup because the passage is only 10 tiles long.

[Reese]

This reminds me of something I've been prototyping.  The kill level is grates, there is a flood level below that, and a pump on the kill level pumps down a shaft into the flood level from a reservoir.  flood floor is used drained to the reservoir.  when the fill pump is turned on, the lava should flood up through the grates, then drain out through them when the drain pump is turned on.  That way there should be only two levels needed (aside from the reservoir)
Right now, I'm prototyping it using water in a sea side fort.

yeah, I start new forts just to try out new traps and stuff instead of experimenting with a "living" fort, I'm weird like that.

[Kanddak]

I like it.

You could replace the fortifications with walls. Armor wouldn't end up inside them, and you'd use less magma to fill the system.
You could also replace the evaporation system with a drainage collector and recycle your magma.

[Derakon]

The trick with using walls is that it drastically slows down the kill rate since the fluid has to path down the twisting corridor. Unfortunately I'm not aware of any type of construction that blocks movement, permits fluids, and blocks flying objects.

Granted I could reduce the occurrence of fortifications in that design; the current iteration is a bit heavyhanded.

[Kanddak]

But none of the fluid winds up in the fortification tiles. I'm not convinced that it will actually fill more slowly.
You could certainly cut it down to three columns of fortifications: one at each end and one inf the middle.

[Quietust]

If you supply the corridor's fill pump from a "pressurized" magma tank (filled by a large number of pumps, preferably at least 10 or so) you could potentially pump one tile of 7/7 magma per frame, which would allow you to fill the entire corridor in about 1 second.

[Atarlost]

You might move the magma intake up a level.  Another level exactly like your collection level (but with a couple of the grates replaced with floors to avoid the need for pillars) that magma pours in and through would let the magma fall from above all over if you pump it fast enough.  You'd need to pump in from several places around the edge so it'd need more power, but it could deliver *lots* of magma allmost instantly. 

[Vicid]

What if there was just a level above the killing floor with retractable bridges?  This level would be filled to 7/7 with magma.  The bridges retract and instantly the magma falls into the killing floor from above evenly.

[Grax]

What if there was just a level above the killing floor with retractable bridges?  This level would be filled to 7/7 with magma.  The bridges retract and instantly the magma falls into the killing floor from above evenly.

Hatches, booze bombs and 3/7 is enough for me. ;-)

[Atarlost]

Hatches/bridges vs grates and lots of pumps is a choice that depends on how much bauxite you have.  If you're importing it getting enough to for just the bridge mechanisms is going to be a pain.