How would you be connected to the outside world? I imagine it as the Depot being on top of the tallest column connected to a tunnel to the surface by a bridge. Also, would your dwarves walk along on the ground floor to get between stalactite/stalagmite/column, or use raised walkways?
Like I said before, the depot is an interesting question...probably on or near the surface is the best place...remember, wagons can only cross z-levels by using ramps and they needs a three(3) tile wide space to move.
I'm going to go ahead and say moving around will be on the floor level AND in tunnels above the ceiling. You need those ceiling tunnels so you can get into the stalactites and columns are a good way to get between floor and ceiling. Said ceiling tunnels can also connect to wherever the depot is and ultimately, the surface. Floor movement is also important, though, for moving between columns, stalagmites, and whatever is on the ground level(farms, whatever).
I'm thinking that the trade Depot will actually be on the ground level (when I refer to the ground level, I mean the bottom of the cavern, probably somewhere around +3 from the bottom of the map). I want visitors to the fort to appreciate the grandure of the design. The entrance to the fort will be a large 5-tile ramp that traverses back and forth all the way from the entrance at the surface. The whole thing will be open, so that as they make the trip down to the Depot they can look out and see the entire town spread out below them.
I suppose I'll have to have some alternate meat-grinder type entrance for invaders somewhere else.
As for walking, I will certainly take your suggestion and have tunnels in the top layers. Rather than basment tunnels, however, the ground floor will be filled with walkways (pretty towercap-lined ones), with doors at the base of each stalagmite. Additionally, I probably will build some elevated walkways connecting each stalag structure somewhere in the mid-z levels. These will be constructions, not carved, in order to go along with the whole "found cavern" feel.
Some other ideas I have:
Perhaps the entrance to the cavern will be inaccessible except for crossing over a drawbridge spanning the surface-exposed magma pipe. If I do this then the caravan route I describe earlier will circle around the pipe on its way down to the bottom floor, like a giant spiral staircase.
Otherwise I might just use scaffolding to smooth and engrave the pipe, or dare some kind of magmafall.
One or more of the stalagmites will be dedicated to metalworking, and the channels that feed magma to them will be exposed to passersby on the ground floor, with steel bridges built over them. Magma monsters don't respawn, right?
I've decided that the main dining hall will be located one level below the ground floor. The entrance will be a lavish gazebo type thing, probably in the town center, on the ground floor with stairs leading down. Statues and mist generators in there for sure.
The dining hall itself will ideally have clear glass walls, with the magma canals visible through one side and the water canals visible through the other.
I think I'll build the dining hall "restaurant-style", rather than "cafeteria style". Booths, private rooms, and clumps of four tables spread around. Conventional wisdom states that dwarves need a mile-long table, but they never seem to throw banquets, do they? I think I've been misreading them; I think they want intimate seating.
Catacombs and burial grounds in a labyrinth in the bottommost levels.
What should I do with the HFS chamber?
Hang on, I saw some people posted while I was writing this, I'll respond in the next post.
EDIT: Added a quote for clarity.