So, here's my notes from making a working magma hallway trap system that preserves all weapons & armor, is entirely immune to building destroyers, but kills -everything- inside and is FAST. (cycle time from first creature trigger to magma free hallway floors is 17-20 seconds, measured by the stopwatch)
It is also entirely automated, including reset. It has exactly one ... annoyance, which I'm not sure there is a fix for, but more on that after the spoiler link!
Requirements: Volcano. Infinite water source(s). Four Z-Levels, and as much space as you'd care to use. My proof of concept example all fits inside a 50x25 square per level.
Important: Channel out below bridges before placing them.
Z+1 (above hall)
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Pressurized magma source: (volcano on the left)
MPPS
MPPSS
MPPSSS
MPPBBB
MPPBBB
MPPBBB
MPPSSS
MPPSS
MPPS
M - magma (below)
P - iron pump
S - smoothed floor
B - retractable bridge (channeled out below the bridge)
This can be setup in any direction, the important part is nine pumps, and that the magma source be as near to the volcano as possible, ideally you'll be drawing from the squares adjacent to the vertical obsidian tube wall that contains the volcano.
Hallway layout (Z+0)
...
SSSBBFFFFFFFFFF%FFFFFFFFFFBB~traps~bait/fortress
SSSBBFFFFFFFFFFPFFFFFFFFFCBB~traps~bait/fortress
SSSBBFFFFFFFFFF%FFFFFFFFFFBB~traps~bait/fortress
^^^
magma dumped here controlled by bridge (acting as a hatch) above, in the Z+1 level.
S - stairs down to the trap hall
B - raise-able drawbridges as walls, 2x3 works fine.
F - retractable bridges as floors, 10x3 max size, need at least two (20x3 total) on either side of the creature pressure plate to ensure capture of titans/fast movers/many goblins
C - channel above to PP2, when the room is completely full, magma will be pushed up this channel onto the pressure plate. Floor on that tile, in the hallway, is a bridge as normal. It's at the end of the hallway to ensure maximum fillage detection.
Z-1 (below hall)
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Floor grates to catch goblinite as it rains down from above, free of its organic restraints. Be free, goblinite!
PP7 in middle, adjacent to the center line below the creature pressure plate on Z+0.
Z-2 (below halls)
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reservoir/catchbasin/drain to map-edge
dwarven logic-path layout
RS1 RS2 RS3 RS4
FG1 FG3 FG5 %%%FG7
PP3 PP4 PP5 TIL
FG2 FG4 FG6 PP6
EDG EDG EDG EDG
RS - river source (infinite water source of some kind)
FG - floodgate
PP - pressure plate
EDG - map edge carved fortification (for drainage)
RS1 logic flow controls wall-bridges in the hallway, to trap the goblins
RS2 logic flow controls magma source above the hallway, to burn the goblins
RS3 logic flow controls hallway floors and collect the organic-free goblinite
RS4 logic flow controls drainage & trap reset
PP1, creature only, 1000 weight min, any max, tied to FG1 & FG3
PP2, magma only, (6 min, 7 max), tied to FG4 & FG5
PP7, magma only, (2 min, 7 max), triggers slow reset, near grates, tied to FG7
PP3, water only, (4 min, 7 max), opens wall-bridges to block hallway
PP4, water only, (4 min, 7 max), opens magma source hatch on Z+1
PP5, water only, (4 min, 7 max), opens floor to drain
PP6, water only, (2 min, 7 max), resets trap (may need to be 3min/7max)
Logic flow:
PP1 triggers FG1&FG3, closing the wallbridges(FG1) and starting magma flow (FG3)
hallway fills with magma, under pressure.
eventually, hallway is full
magma full triggers PP2, which is tied to FG4&FG5, turning off the magma flow and removing the floor
magma drains below, triggering PP7
PP7 raises FG7
water flows (slowly) to PP6
PP6 raises FG2 & FG6 to drop the walls and return the floor to normal.
mechanism ties (in order)
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lever1 -> bridge-walls in hallway (x2)
lever2 -> magma-source-hatch-bridge (x1)
lever3 -> floor-bridges in hallway (x4 at least)
PP7 (magma drained indicator) -> FG7
PP6 (logic) -> FG2, FG6
PP4 (logic) -> magma-source-hatch-bridge (x1)
PP5 (logic) -> floor-bridges in hallway (x4 at least)
PP3 (logic) -> bridge-walls in hallway (x2)
PP2 (magma full indicator) -> FG4, FG5
PP1 (creature) -> FG1, FG3
The levers are there for dealing with titans and whatever else you'd care to manually trap and melt. They're also handy for testing as you build. Basically you just flip lever1 then lever2 then lever3 to empty then reverse to reset.
While I did floor-bridge + floor-bridge + creature-pressure-plate + floor-bridge + floor-bridge, what is actually better is floor-bridge + creature-pressure-plate + floor-bridge + floor-bridge + floor-bridge (creatures entering from the left) as this allows for three bridges worth of travel time before the hallway is sealed. With only two bridges worth (20 tiles) it's a little tight on the timing with single-file trolls, for example. They often get right to the edge before the bridge walls go up. A small detail, but worth mentioning. My next version will be 20 tiles prior to and 30 tiles after the creature trigger, to ensure maximum efficiency.
The only slightly annoying thing about the system is that there is a single 1/7 magma often leftover on the creature-driven pressure plate that blocks pathing for the evil monsters. So, they COULD come in again, but they stand around doing nothing waiting for one single tiny 1/7 magma tile to evaporate. Pumping from above does no good as 1/7 magma cannot be pumped. If anyone knows how to speed up magma evaporation, please let me know.