I thought, for a moment, it was more like:
SIDE VIEW:
(repeat as much as necessary, as tall as you want, and hell, even stagger it a bit more;
leaving a clear path between staircases for crossbowdwarves to shoot through)
Z+3 X^X X^X
Z+2 X█X X█X
Z+1 X█X X█X
Z=0 X█X X█X
Z-1 X█X X█X
Z-2 X█X X█X
Z-3 X█X X█X
Z-4 ^X█X^X█X^
Z-5 █████████
Z-6 ==> Ballista waiting below
Z-7 ^^^^^^^^^ <== Another ballista waiting below and/or trap gauntlet
Basically, they're a series of up/down towers with staircases lining 2 sides of them, as to not allow the invaders to path around the bend; between and on top of each tower is a trap (mainly corkscrews and disks), and the towers are of sufficient height, and falling off and into the surrounding pits that railroad the invaders through the death towers and force the invaders back through the loop, and all the way to the beginning of the gauntlet.
And like suggested, these will be in front of the fortifications, shooting from the side, so the archers have something to shoot at; in the underground pits that rally the invaders back to start, you can mount the ballistas to clear the pipes of the fallen invaders as they scurry back to start.
So, something like this (TOP VIEW): (All Z combined; reference with SIDE VIEW for Z-levels)
██████████████████████████████████████████████████ Z+5 or whatever
/▀▌╬ ^X^X^X^X^X^X^X^X^X^X^X^X^X^X^X[u]++++++++++++++[/u]+ Z-4-Z+3
»=>╬ ^ (Return channel where ballista shoots)+++▲▲ Z-6
\▄▌╬ X^X^X^X^X^X^X^X^X^X^X^X^X^X^X^X… ▲ Z-4-Z+3
+++█ (Trap-laden return channel)^^^^^^^^^^+++++ Z-7 (alternatively, you can also place a ballista here)
████╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬╬████████
++++++++++++++++++++++++++++++++++++++++++++++++++ Z=0
Crossbowdwarves can shoot between the towers (hitting front and back tower stairways), shredder traps can force invaders to dodge into the pit, or on top of the lower trap; bolts can also force invaders to dodge. Invaders in the pit are welcomed with a ballista bolt down the channel as they bolt like a rabbit out of a viper's pit.
This setup should be cheap and easy to build or mine out, and ought to be entertaining. This was at least my thoughts on using the vertical to your advantage; or at least, what I initially thought was your idea. I don't mind adding this as a security feature for my fort. After a prospect of my region, it turns out I lack the necessary materials to mass-produce good military defenses, so this would make for a good replacement for it, via trap-style and shooting range. In other words, expect to see this feature put into action when I continue my fort. I'll upload a clip if I get it working.
EDIT:
Come and think of it. It can just be staircases, and a floor bridging the gaps between them in alternating style, with a trap on each of them. This leaves even more room for the crossbowdwarves to hit their target with more ease. Just make sure there's nothing between the bottom-most portion, unless headed to the next "tower". Keep the pits -2Z from the bottom-most level as to forbid invaders from shortcutting.
Oh yeah, in the future, [ tt ] (minus the spaces, teletype/the little typewriter icon), along with pre-building it in Notepad, is helpful for making properly-aligned maps for these kinds of setups, like I did.
EDIT EDIT:
Same thing, but without walls (even cheaper material-wise to build; more practical for crossbowdwarves to fire through):
Z+3 X[u]^[/u]X X[u]^[/u]X
Z+2 X X X X
Z+1 X X X X
Z=0 X X X X
Z-1 X X X X
Z-2 X X X X
Z-3 X X X X
Z-4 ^X X^X X^
Z-5 ██ ███ ██
Z-6 ==> Ballista waiting below
Z-7 ^^^^^^^^^ <== Another ballista waiting below and/or trap gauntlet
One more idea:
Build these, and have ballista built on each level, and fire at them in a straight line as they charge through. Seeing as there are no walls to worry about anymore. Why not?
/▀▌╬
»=>╬ +X^X^X^X^X^X^X^X^X^X^X… Z=0-Z+7 (Invaders come from here) 7x Ballista 1-per level as well.
\▄▌╬ + (Into the base) Z=0