Would it be possible to build a Moses-o-tron style entrance to a fortress, with magma to each side and pumps that constantly keep a central path clear, but a switch to send waves of magma down on a siege? Here's what I'm thinking:
g g
Wm%% %%mW
Wmmm mmmW
m=magma, w=wall, %%=pump, and g=Gear assemblies connected to a power supply and an on/off switch.
I'm not sure if this would work however, or how much magma it would need.
*musing*
What if the whole thing were to be constructed on the principle of a dead-man-switch?
Somewhere in the hall there is a confined creature that intruders want to and can kill.
Of course, this trigger area (or prison) is to be made safe from magama, putting drains before it or the likes.
Might be tricky if the creature is to be killable by melee but that's a technical issue.
The creature's aliveness keeps pressure plates triggered regularly.
When the aliveness becomes absent (from intruderish action), the pressure plates are no longer triggered.
The pumps stop working.
There is a sudden silence in the room.
Followed by many very loud (but brief) screams.
The obvious advantage: the intruders
intentionally trigger the trap themselves (they just don't know that), making it far more satisfying than a mere bangyouredead trap.
Only trouble is, you'll have to reload it manually, putting a new creature into the prison.
But who knows? Maybe a better dwarven engineer could design a bait magazine that releases a new creature into the trigger area.
That would mean that the trap can reset and fire repeatedly during a single siege without supervision and every time it can toast-a-host of enemies at once.
Hmm. Toast-A-Host
tm trap. Sounds like a marketable name!
Bonus points for letting the magma fill the hall from both ends (exits), letting the intruders watch the walls of fiery death close in on them.