I'm going to try this.
A 6x6 embark in the EXACT centre of a region tile. The initial entrance building will be 10 z's high, and will be shaped like a crude pyramid. The top will be 10x10 tiles with a 2x2 set of stairs leading down one level into the pyramid. This top level will have barrels full of food and drink to survive the harsh, monstrous, trap-filled environment of the dungeon. In the sub-ground regions, the rewards are better (gems, armor and weapons), but the risks are greater (corkscrew traps, arrows coming out of the walls). Above the caverns, when one gets drowsy, there will be 3x3 'safe rooms' with a bed, some extra food and drink, and place to drop your possessions. It is then possible to sleep. Once you wake up, you can gather your possessions, leave the room, and continue. Once you hit the caverns, a set of fortifications will be placed along the fort's edges in the caves, with raised drawbridges (and the levers to raise them being hidden in a sealed room), one per side. Creatures can hopefully get in, and get out. A solid floor will lead you to where the way to the next cavern layer is. You may wander off the trail, for useful items, tools and other trinkets lie, but keep in mind that dangerous traps such as Giant Cave Spider (GCS)-Cage Bombs will be strewn throughout the level. Pressure plates will set those off, and you will have no idea what just happened. After the caverns, you will be warmed (potentially uncomfortably burned) by the surrounding Magma and the Semi-Molten Rock (SMR). Passages to solid rock down deeper will be guarded by potentially lethal traps (10 Adamant spears/spikes per trap, compared to the one training spear above-ground, 4 bronze spears above the caverns, 7 steel spears in them), caged war animal cage bombs (war bears). Once (if) you manage to get into whatever way into Hell, the final prize lies there: All the artifacts you can get your hands on. In a few rooms on each dungeon level, there will be bins of ammo, weapons, and some random assorted monsters. In rooms in the deeper levels (SMR levels in solid rock), there will be high-quality weapons and armor, trap components, and lots of trinkets: Valuable gems--normal and large--, high-quality crafts and instruments made from rose gold, platinum, bright silver, orihalcum (Civ Forge), etc, and bars of many different metals that, added with Wanderer's Friend, are workable into a _very_ limited amount of weapons.
This, obviously, would take an INSANE amount of stone, a good bit of wood, and lots of workers. For me to build this, anyone with no stone-working skills (mason or engraver) is automatically conscripted to be a mason for building the walls and an engraver for smoothing the walls of levels above the caverns, and for engraving the walls and floors of rooms and halls in the Magma Sea. Anyone WITH stone-working skills are automatically conscripted for mining service to dig out the rooms and the stone to make the pyramid.
My apologies for the long post.