Cheers for the necro. I've got a good excuse to mention that I'm using inverted pillbox-towers in my current fort.
Eventually, the land beneath these will be channelled away to many, many levels, so before I get round to that I've been doing the following, slightly convoluted method designed to avoid dumb-builder trapping, circumvent cave-in possibilities (if done right), remove the possibility of even momentary hostile-by-the-back-door access, uses a different method of tethering the animal (with option to stop them running away or not), gives maximum view-distance (i.e. same Z-level)
and you don't get the un-aesthetic knowledge that beneath the built walls there technically exists the soil floor it might have been built on...:
Z-1 Z=0 Z+1
####### .......
####### .......
####### .......
####### ....... Start on your flat grassland...
####### .......
####### .......
####### .......
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
###^### ...v... Dig a channel
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
###^### ;;;<;;; . Build an Up-stair
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
###^### ;;;v;;;
##^^^## ;;v<v;; . Dig some more channels
###^### ;;;v;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
###^### ;;;H;;; .
##^^^## ;;H<H;; ... Build fortifications (windows possible?)
###^### ;;;H;;; .
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
##^^^## ;;vHv;; .
##^,^## ;;H<H;; ... More channels
##^^^## ;;vHv;; .
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
##^^^## ;;HHH;; ...
##^,^## ;;H<H;; ... More fortifications
##^^^## ;;HHH;; ...
####### ;;;;;;;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^^,,^# ;vHHHv; ...
##^,,^# ;;H<Hv; ... Nearly separated with channels
#^^,,^# ;vHHHv; ...
#^^^^^# ;vvvvv;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^^,,^# ;vHHHv; ...
##^,,^# ;;H<Hv; +++.. Floor along from the upper floor, just the edge
#^^,,^# ;vHHHv; ...
#^^^^^# ;vvvvv;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^^,,^# ;vHHHv; ...
##^,,^# ;;H<Hv; +++>. Down-stair from above
#^^,,^# ;vHHHv; ...
#^^^^^# ;vvvvv;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^,,,^# ;vHHHv; ...
#^,,,^# ;vH<Hv; +++>. Dig away the last tile
#^,,,^# ;vHHHv; ...
#^^^^^# ;vvvvv;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^,,,^# ;vHHHv; ...
#^,,,^# ;vH=Hv; +++>. Set a 1x1 pasture in the pillbox
#^,,,^# ;vHHHv; ...
#^^^^^# ;vvvvv;
####### ;;;;;;;
####### ;;;;;;;
#^^^^^# ;vvvvv;
#^,,,^# ;vHHHv; ... Put something non-foraging in the pasture
#^,,,^# ;vHtHv; +++c. (e.g. a poult, of which I currently have a million)
#^,,,^# ;vHHHv; ... and a hatch on the top stair (which can be set to be pet-impassible
#^^^^^# ;vvvvv; if observer is completely expendable and needs to be kept there!)
####### ;;;;;;;
Key, in case you can't work it out (and I haven't made any errors!):
# = raw soil ; = raw surface , = exposed ground
^ = up-ramp v = down-ramp . = something below
< = up-stair > = down-stair H = fortifications
= = pasture + = built floor c = floor hatch
t = turkey poult
Alter to taste, and the building usually ends up being done a bit more asynchronously. Also, the Z+1 access floor often travels onto/between other similar pillboxes and gets itself lined with walls (for protection of all dwarves) and/or further fortifications (for ranged sniping down from, although the hanging pillboxes themselves could also be used for that, if and when it all kicks off), as desired.
Yes, it
is a lot of work. Especially if you insist on every bit of the thing (including walkway) being made in marble blocks, like I currently am doing.
(OTOH, it has detected ambushers,
plus easily survived a siege (everyone stayed inside, without any need to use alert burrows and instead just by dint of no-one needing to exit the inner compound...
eventually the siege walked onto my bridge entrance with doubly-isolating ingress/egress where I could trap them and use them for target practice.)