I want to build an iron tower, stretching from the entrance level down 6 or 7 Z levels (obviously limited by my iron deposits). Probably 7, so that each of my founding dwarves will get a level. There'll be a dining room on top, and a moat around the bottom, with a well inside the lowest. Might even have plumbing, providing a watersource on each level.
The floors will be made of whatever expensive material each founder likes, as will the inner walls. The only entrance will be from a single door in the base, though I may have a secret escape tunnel dug for them.
It'll stand alone in a large chamber, possibly stretching to the sky, with a clear glass ceiling over it.
I also want to have another tower for the nobles, similar but hollowed out. They'll have to cross a retractable bridge at the top to pull a lever, which will plummet them to their doom. It'll be made out of the cheapest, most common material I have, not smoothed, and have a bridge at the top for an entrance. I'll also dump goblins down the shaft, and other refuse.
If the nobles somehow survive, they'll be locked into a tube filled with rotting corpses, and likely too crippled from the fall to do anything about it. In fact, I'll probably build it at a height to have a better chance of a non-fatal fall.