Completed the first and basement level of my tower last night. It's loosely based around Orthanc (Saruman's tower from the Lord of the Rings) so I'll just call it that for convenience now: http://www.shackpics.com/files/Orthanc_3_bwzeb779afvitrylg8xm.png (note that any dirt inside the tower is temporary scaffolding).

Some things I have learned so far:
1) Stair code is still buggy as shit in MP. I think what happens is, if you place stairs with any ambiguity about which direction they're supposed to go (i.e. usually if they have 2 neighbouring blocks), they glitch the fuck out and you rubberband like a mofo when you hit that stair. Follow Arcanum's directions for stair placement (check the wiki) and you'll be fine. (This is why there are floating bits of dirt to the side of the staircase in the above pic).

2) If you want a quick way of clearing gravel, clear out everything below the column of gravel except for one block, and put a torch under that block:

GGGG
GGGG
GGGG
SSSS
TTTT

G=gravel, S= stone, T = Torches

Then knock out the stone blocks, the gravel falls onto the torches and becomes collectible blocks: you don't need to dig it out.