I think I'll compromise and have a 5 z high tower, but firing from level 3.
The self sustaining tower idea is pretty cool, but not really what I'm after here, although I may well use the airlock idea for a storeroom so that supplies can be brought while keeping my marksdwarves locked in.
So at the moment my underground layer will look something like this (not to scale):
MMMMMMMMMMoMMMMMMMMMM
MMMMMMMMMMoMMMMMMMMMM
MMMMMMMMMMoMMMMMMMMMM
XXXXXXXXXXoXXXXXXXXXX
XXXXXXXXXXoXXXXXXXXXX
XXXXXXXXXXdXXXXXXXXXX
XXXXXXXXSSSSSXXXXXXXX
XXXXXXXXSSSSSXXXXXXXX
XXXXXXXXSSSSSXXXXXXXX
XXXXXXXXSSSSSXXXXXXXX
XXXXXXXXSSSSSXXXXXXXX
XXXXXXXXXXDXXXXXXXXXX
XXXXXXXXXXwXXXXXXXXXX
XXXXXXXUoLULoUXXXXXXX
XXXXXXXXXXoXXXXXXXXXX
XXXXXXXXXXoXXXXXXXXXX
XXXXXXXXXXUXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXX
M = Main fortress
X = Rock
o = Mined out path
D = Door controlled by lever 1
d = Door controlled by lever 2
S = Stockpile
U = Upward stairs
L = Lever
w = well
Stockpile contains food, booze, wood, metal and ammo, taken from their respective piles in Main Fortress.
Z lvl 1 contains (in each tower) Dining Room and booze and food stockpiles (taken from airlock stockpile) (3 wide dry moat around the complex of towers and two drawbridges on either side of frontal tower leading to main entrance)
(Not to scale and ignoring L shaped entrance and traps etc after this))
XXXXeeeXXXX
mmmXXXXXmmm
mmmDmTmDmmm
mTmDmmmDmTm
mmmDmTmDmmm
mmmDmmmDmmm
X = Ground
m = Moat
D = Drawbridge
T = Tower
e = Entrance to main fortress
Z lvl 2 contains barracks, archery practice room (with arrow recovery channel) and ammo stockpile (Taken from airlock stockpile)
Z lvl 3 has fortifications from which to fire and an ammo stockpile (taken from the one downstairs)
Z lvl 4 has 2 bedrooms for the civilians (gotta be able to make arrows in times of need)
Z lvl 5 has a wood/metal stockpile and the workshops required to make arrows from them
Hows that sounding for everyone?
And a new question. If for a later project I decided to make the moat floodable / drainable, can dwarves aim through waterfalls? It would be AWESOME to have waterfalls coming down the outside of these towers.
Ooh, and would this in any way be possible with magme? That would be the ultimate in dwarfy!