Tosid and Unib Keskalerib stood on the packed red sand floor, looking out over the pit. Rising out of it were eight tall polished steel pillars, masterfully carved grates held tightly to their sides by intricate clockwork. The pit was easily wide enough to swallow a wagon, and stretched the full length of the main entranceway. Below the surface, sheer dolomite walls fell thirty feet to a smooth dolomite floor, the carefully-fitted white tiles blending invisibly together into a smooth white plain.
"All I'm saying is that it was a slight miscalculation." Tosid was saying, "The pumps don't drain fast enough, so the water backs up and they stay open. I already know how to fix it, just a matter of a few more mechanisms."
"That's all well and good," said Unib, the mayor of Lanternwebs, "But when can we use the entrance again? How safe can we feel, knowing that the floor might drop away at any moment?"
On one of the eight steel pillars, a cat hissed into the gap, its back arched as it glared down at the floor that had suddenly abandoned it. On the floor below, four hundred and twenty tiles, indistinguishable from any of their neighbors, snapped back with blinding speed as a forest of polished steel spikes rose quickly from the floor, impaling an imaginary horde of trapped invaders. The cat leapt back from the edge of its pillar, barely avoiding a long fall off the opposite side.
"Oh, don't worry about that." Replied Tosid, "As long as the main floodgates are closed, so should the grates. I've sent someone to pull the lever and close them, and the water should be drained any moment now. They're perfectly solid, you've been walking on them for years now."
With a snap, obsidian grates swung upwards from their positions along the pillars and outside walls, covering the pit in a checkerboard of black stone and empty space. Far below, the spikes withdrew into the floor as quickly as they had emerged. A few whistles of appreciation rose from the tunnel to the barracks, where training had been paused at the noise.
"I may have been walking on them for years, but I didn't know they were liable to disappear at any moment!' Unib said angrily. "What if one of your mechanisms should fail, what then? A dwarf dead because you needed to convert the main entrance into a, a, a deathtrap!?"
The cat pawed the stone grate cautiously, as if expecting it to change its mind and suddenly disappear again. Eying it suspiciously, it placed its front paws on the surface of the grate just as white dolomite grates snapped upwards to fill the empty spaces in the grid. Again, the cat lept backwards at the sudden movement. The grates seemed to provide a solid floor, though, and the cat quickly scampered across them to the safety of the red sand.
"Nonsense! I'd never make a mechanism that could fail." Tosid responded indignantly. "Besides, they've been hooked up to the mechanisms for a good six months already, I just needed to seal up the plumbing and find a good time to test it. No accidents so far. Just tell everyone to stay out of the spike pit and away from the entrance if we ever need to use it during an attack. I'll have them opening and closing like they should in no time at all, don't you worry."
Somehow, thought Unib, having them open and close the way that they should wasn't any more comforting than having them open and close at all.
-------------
Phase one complete, aside from the minor tuning to get it to alternate correctly! Once I get that in, I'll put up a movie of it.
A technical diagram: Technical diagram replaced with screenshot.
The entrance corridor (a 5x10 area) is now fully rigged with grates that, with the pull of a lever, will open and close in such a way that there will always be a path into the fortress, but never a "safe" path: all of the Obsdian grates are linked to one trigger, and all of the Dolomite grates are linked to another, so that only one set is closed at a time, and they alternate as fast as grates trigger (10 step delay). So, an enemy attempting to cross the entrance will be forced onto the currently-solid grates, which will toggle at some point during the crossing and drop them into the spikes, which completely cover the bottom of the pit, and are also hooked up to an alternator so that pulling the lever causes them to cycle up and down until the lever is deactivated. The pit and the beginnings of the spikes can be seen in the images back
here, and once I remember to do it I'll be uploading a current map to the DFMA.
As if that weren't enough, the only exit from the pit leads to the barracks, and there are fortifications in the second level for marksdwarves to shoot into the pit (if, for whatever odd reason, everything in the pit isn't dead from the spikes).
Given that I'm quite happy with the military's performance thus far, it's unlikely that the deathtrap will ever see use in anger. I had originally designed it when I wasn't sure if it would be practical to raise an army that could confidently engage the orcs, as a horribly
overcomplicated dwarven deathtrap to deal with the sieges (or at least stun them with the drop to give my military an edge). Somewhere in there Tosid went secretive and I decided to throw in a bit of fluid logic and insanity, just for fun, and keep building it even after the military proved that it could fight the orcs on even footing. I'm quite satisfied with the results.