The originally-conceived megaproject was a fountain, but the design of the fountain looked very much like a Mayan pyramid, and I remembered reading in the forums about people using Mayan pyramid like structures to dispose of captives, and, well, there you go.
The pyramid was cast from obsidian, with the magma provided by what would become the hammer, and the water provided by what would become the pick. Each level was done with this process:
1) Build a mold. For the lower levels, this was a two-level mold; for the larger levels, one level was all that was necessary.
2) Flood with 2/7 magma.
3) Flood with water. In the case of a one-level mold, excess water spilled into the aquifer.
4) Remove mold (again, water drained into the aquifer)
5) Repeat, using a new, smaller mold on the new level.
The first two levels went slowly; the rest of them were relatively quick. *Everyone* was given the mason task. for building/removing the mold.
The accent on the pyramid is cinnabar, which I found metric tons of during the exploratory mining. The statues, in addition to being a nice artistic touch, were put on to correct the design flaw -- I was unaware of the fact that you could use a ramp to move up *and* diagonally at the same time until I saw my dwarves do while smoothing the pyramid, so the statues are put in place to keep someone from using the ramps in the death channels to escape and get onto the pyramid proper.
The death channels that the captives must travel to escape are filled with ten weapon traps each, one per level. Each trap contains ten silver weapons: two swords, two maces, two spears, two hammers, two axes. Grand total of 320 weapons.
Why silver? Because the idea of death-by-a-thousand-cuts amuses me (what fun is it for the captives to die on the top level?), but also because by the time I was putting in the traps, I knew I was essentially out of iron *and* copper.
Sadly, I received no sieges nor ambushes once it was completed, so it as yet untested as to whether it is easy or difficult for a captive to escape alive.