(It's perhaps easier to use the [
code] tag, or <
pre> (which allows color-tagging) to show ASCII-plans, both side view and top-down. Maybe force [
font=courier] if you can also ensure by the <
pre> that it does not collapse whitespace or avoid it doing so by doing solid blocks of non-space characters. Though not all browsers do that perfectly, it works better than presuming your proportional font adjustments to spacing apply the same to everyone else.)
I like doing multi-level walls around areas (high enough to enclose resource-trees) with a retractable roof that is built entirely from within (once completely enclosed) and leaves no pesky entryways to fliers or climbers. And that was before there
were (deliberate) flying or climbing hostiles to worry about.
To cap off the top I tended to do it with bridge-roofing accessed from two points, under two different sections of retracting bridge (needs a built lever for each separate retraction mode you eventually need, and a quantum-connection pair for each unit needing retraction). At the last level of internal wall I build a ramp, instead of wall, on an edge (not corner!) tile, which might be where any internal stairway access meets the wall or just anywhere the internal wall-hugging 'gantry' is there, for the express purpose of patrolling archers[1], or however you are currently with the entire level eventually floored in a stack-of-rooms tower design.
These two (or more!) ramps give access to the final wall-tops. Now
consider how you're going to bridge-cover the gap. It must have separate bridges to cover each ramp (of the two key ones, regardless of others you temporarily have), the maximum size is 10x10 and all bridges must be buildable from non-bridge support. Anything up to 20x20 plan (including walls, i.e 18x18 internal space) can be done with at most four bridges anchored on each corner and half way along each wall (any side less than 10 can have two corners, two complete sides and as far as you want/can along the adjacent sides, and if it’s >20 in the other direction you can fill across between opposite sides without further need to worry). And if you have internal walls in the level below the roof then you probably can handle it as several smaller sections anyway. (You don't need to, nor necessarily should you, tie the roof-bridges to the support-walls
exactly, just treat those wall-tops as
potential roof-gantry flooring (below), gratis, in your subsequent planning.)
If you're doing anything of 21x21 or more, without any other inte, you'll need at least 3x3, and the centre one would need support. Optimally, in each direction, (Walltop-supported Bridge)(Bridge)(Floor Gantry)(Bridge)(Bridge supported by Walltop) would work, with the Walltop being below the edge and the Floor Gantry being constructed (or naturally arising from internal Walltops up from the innards of the level below) from external walltops all the way across the given perpendicular. You could also have an 'island' in the middle, but then it needs its own access anyway.
(In a 3x3 bridges of <=10 on each side, you could go (Bridge)(Bridge)(Gantry)(Bridge), if you don't mind assymetry, or are copying the design at each corner of the map anyway so you get your symmetry when considering them as a group!
)
Anyway, get your gantries built between your walltop edges. Then lay down all bridges but
one, that one being one that overlaps a chosen wall-ramp access to let you build the others. Once the bridge(s) covering the other ramp(s) are built (make sure they're first in the job queue by holding back on others?) and blocking them, connect the lever you built in your Master Lever Control Room/wherever with this(/these) blocking roof(s) and - making sure there's no construction-dorfs likely to be wandering over them on the way to other bits - pull the lever. Now you can safely a) unbuild the first access ramp in order to rebuild as a wall, and b) set the roof-bridge to be designed and built over the gap you left for it.
Any further roof-bridges that need to be lever linked (including this last, if that's your wish/requirement) can be done with the currently unblocking bridge open and then when you're finally done on the rooftop and nobody is conducting any further business up there, pull the lever(s) to unretract the retracted bridge(s) and rereplace the last ramp(s), now blocked from above, with the required wall(s) and you're sealed beneath a roof. Regaining access, from inside, needs retraction of at least the one bridge and deconstruct a handily revealed wall to (re)replace with ramp again. I've been known to station vampires up there, in the old days before climbing was a thing, just because I could set them to patrol around and
maybe spot something whilst doing so, without caring for their needs or even their fate.
Or build a stairwell up in the middle into a 1z-higher 3x3 walled chamber with side-access to roof-level (set a ramp up its outside to roof
that over with 3x3 flooring at the z+1 from the main roof) and use it as access to climb out and seal the main roof area properly in any form you see fit to (as long as it covers all your edge walltops). Either wall up the gap you used to access the roof or make it a raising bridge for handy togglable portal, and you can reverse this particular seal the next time you want to access to the level (a new phase of expansion, making it even higher?)...
[1] If including Fortifications around a wall edge, I find it better to build as Wall then carve the wall into Fortification. This preserves the wall-top, whereas building as Fortification from scratch does not give an under-built floor on the level above, complicating some forms of further building upward.