I didn't see this thread back in June, so I'm a bit late to the game in describing how I'm currently setting up my observation posts (like Archery Towers, and usable as such, but currently populated with juvenile fowls, of various kind, as ambush-revealing guard-creatures.)
_________=_____#
Fortress <____=#
innards # H<H
__________#___________
#=Wall, H = Fortification, _ = notable floor, < = Up-stairwell, = = Hatch (over down-stairwell)
From the wall, and without any direct path to the outside (although still vulnerable, while building, to flyers), I'm building wall-lined walkways leading to hatch-sealed downward stair into the "bellygunner pillbox" in which the creature (and, as necessary, possibly an on-call archer) resides.
Building necessitates getting the lower level built first (in the above example, from the first floor[1]) with constructed floors and then an up-stairwell (or several) in the required position and surrounding it (or them) with Fortifications (using bridges or more floorways[3] to build the corners, and then when they're secured by other adjacent walls deconstruct the given walkway and replace with that wall's fortifications), keeping the supporting 'access' floor until the second-floor walkway has been constructed enough to 'latch on' to the structure, then gradually deconstruct the temporary lower-level access floor as the down-stairs (to be covered by a hatch, to make forbidden and pet impassible while the juvenile birds reside in them, to avoid certain auto-hauling and enemy-fleeing tendencies) make it possible to fill the gap with the last fortification. The level above is wall-lined as and when material is available[4] and a stairway to the roof (the top down-stair being givne the potential to be hatch-sealed, of course) allows the top of the walkway to be protected.
I've also developed my own (probably not unique) way to make diagonally-extending walkways, that end up fully-sealed, without externally scaffolding them at all:
1) From the start corner to the end corner (located at a tile with an exact
45-degree angle from the former) build a zig-zag of floors and designate
walls within the corners. (With a little (B)uild-(C)onstruction-(?)Whatever
magic, including cancellations of specific crowd-placed items and
re-designation of a different component, you can get it all designated in
this pattern ready for them to complete construction with very little extra
micro-management to that you spent a few minutes originally setting up...)
XO
+++
O+O
+++
O+O
+>+
O
2) Make those internal-corner walls into chevrons/brackets of walls, and add
some more walls as corners to the destination spot. (Noting that you could
set this up in step (1), but separated here so as to allow minimal support
to the downstair, as described further above, a little bit more quickly.)
XOO
+++O
OO+OO
O+++O
OO+OO
O+>+
OOO
3) Remove the unnecessary floors, noting that all walls and floors (and
stairs) that remain support all other walls and floors further out on
the limb, and that walkway passage is still possible.
XOO
+ O
OO+OO
O + O
OO+OO
O >
OOO
4) Place walls into all those gaps, to form homogeneous walls around a walkway
that can still be traversed.
XOO
O+OO
OO+OO
OO+OO
OO+OO
OO>O
OOO
...Finis
[1] 2nd floor, to you left-pondians, i.e. the one at Z+1 relative to ground level[2] (often 'first floor', to you) in the above diagram.
[2] Though my first experiments in the system were of creating these "bellygunner" structures over excavated ground so that they could "hover" at ground-level over an ever-deepening pit, with the post-construction walkway being at first-floor level, rather than second-floor to support the first-floor fortification room.
[3] Deconstructed bridges return their material to safe-ground to one side of where they hovered over mid-air, but deconstructed floors drop their materials onto the floor below. Given I usually arrange for it to be safe to recover, at various times of the dwarven year, I don't mind dropping the occasional stone below and cluttering up the place until I can bring it back into the fold.
[4] I tend to want to build in blocks of a particular stone. A current fort has an main fortress area of sandstone blocks, but dacite blocks form the entirety of the extending bellygunner emplacements and their gangways at all points (and levels) beyond the extremities of the fort. I'm actually more short on sandstone than dacite, mostly because I discovered an aquiver running through the sandstone deposits I was going to mine and enblockify.