I started up this challenge because it sounded like a great defense structure for fort defense challenge and the evil race siege supplement mod.
Embarked on a prime volcano spot directly next to a caldera for multiple magma flood traps. Tower itself is 10x10 and only 3 z levels of construction above ground so far for a quick depot, but it spans nearly 100 levels below ground until it reaches the first cavern.
At the top near the entrance is a draw bridge to a central staircase for a quick access to the hive
once the drawbridge is raised invaders are force to take a spiral staircase downstairs through single wide hallways of ballista arrays, weapon and spike traps, magma floods, and cage traps.
After 45 levels of defense the hallway finally arrives at the airlock, which is on top of 4 levels of barracks.
From there it goes barracks -> mastercraft armory-> craft industry -> food industry -> farms -> food stockpiles/meeting hall -> dorms/hospital -> well -> trash weapon/armor stockpile -> magma forges
I did cheat and hollow out a microcline vein for some above ground stone, and in that vein I've been putting all my refuse/coffins. When the tower gets big enough I'll move the mauseleum inside and have a smaller refuse pile with more aggressive dumping into the magma forge well.
Oh, and when I'm done with I'm going to finish channeling around the walls and drop the entire thing into HFS