I DID build a giant tower from Obsidian as a goblin.
My layout:
The main tower should have enough room to hold two "shift-rooms": a room created by holding shift and expanding it left or right. It has either a 10x10 or 11x11 interior. A 2x halway seperates them going only one direction, and they're surrounded in a 1x hallway all the way around, with a spiral staircase at the back end of the 2x hallway away from the side the enterance is.
I surround the rooms in a 1x hallway so that I can carve fortifications into the walls and place archers up to rain down death on seigers.
Depending on how much space is in a shift room, your fort should be either 29 or 31 wide, and 16 or 18 long, with a 2x2 add-on for a stairwell at the end of the central hallway.
For the wide side: Outside wall, 1x Hallway, Interior Wall, Shift Room 1, Interior Wall, 2x Hallway, Interior Wall, Shift Room 2, Interior Wall, 1x Hallway, Outside Wall
For the long side: Outside wall, 1x Hallway, Interior Wall, Shift Room, Interior Wall, 1x Hallway, Outside Wall
Here's the level system for a tower I built on a map that had a volcano and a river.
Levels:
z4minus - flooded with magma for smelters
z3minus- smelters
z2minus - farms
z1minus - baracks, armoury
z0 - gathering area: trading depot, meeting hall
z1- food stockpile. for this level, I combined the two shift-rooms and the hallway into one room.
z2 - specific gathering areas: dining room, hospital
z3-10 - stockpiles for various goods, materials, and metals
z11 - burial rooms
z12+ - bedrooms, offices, etc.
This is not the tower I would build if I had a second go. I would build an entry tower that held the military, and that tower would be connected to two other towers, each accessable through the entry tower. One tower would hold stockpiles, the other would hold goblins.