BaseThe doorway to the west is where dwarfs get in. That doesn't lead anywhere during a siege, in which case invaders have to plow through the cage traps on the north side to get into the tower. All that empty space could eventually be filled with spike traps, but that isn't necessary.
Level 1The dwarf entrance leads to that bridge. During a siege all the tower bridges raise, forming walls. Invaders that got past the cage traps have to run around the perimeter of weapon traps to get to the next stair leading up.
As you can see, I'm not done filling it with traps. The first layer is all cheap weapons, 2-4 in each trap, with mediocre mechanisms. I want it to soften them to prolong their suffering, because spite.
Levels 2+All the middle levels look like this. That bridge allows trap installers/cleaners to quickly access the level without having to spiral up or down. Obviously those bridges raise.
TopThe top level is what invaders are after: the entrance to the fort. Here is where the best traps will go, each with masterwork mechanisms and a full 10 steel/glass masterwork trap weapons. Once the top layer is done the wealth of the fort will be raised by upwards of 100,000 urists so I'm expecting sieges a-plenty. The statue at the elbow there is both a plug (they don't need to path that space) and bait for building destroyers. I'll be replacing the path to the door with another bridge so I can drop invaders down 12 z-levels for the lulz.
Speaking of which, yes that empty space is there so they can fall if they dodge the wrong way. The walls around the stairs can be replaced with fortifications to take care of something that makes it past the cage traps and can fly or gets to ignore weapon traps for some reason.