I have
loads of info ready to spout forth regarding
higher-
level construction. As you can see.
But to build a wall across a gap in a wall-top where a drawbridge (or door, or just a doorway) exists below, without any intention of flooring,/bridging over the 'ceiling' either/both side of thr gap, just ensure that you ask the wall to be built over the gap (
and let it happen) before you ask for a wall to be built upon whichever adjacent wall-top(s) you might get that wall built from. It's just a matter of knowing how your construction jobs get 'serviced' and either planning ahead (if you can) or putting in temporary remedial works to deal with the newly-desired extended building plan (if you are adding to a structure and don't want to unbuild/new-build/rebuild things).
With a two-wide gap, that means that
each end-wall must remain unbuilt-upon (plus accessible to any suitable dwarf and material) while each of the two gaps are covered, then as each gap-coverer is completed you can (usually, give or take other access issues you might give yourself) build the ealls atop the wall-ends below.
A three-wide gap (or wider), as may be found in an intentionally Wagon-accessible doorway, requires careful building and (if you didn't want it to remain, afterwards) unbuilding of floor or bridge to allow access to build the tricky middle walls from. I usually deliberately choose a differently-coloured stone for such 'scaffolding' so that I can see what I intend to remove.
If building off the side of a bridge, though, realise that a bridge does not support a construction. A line of floors can have an equivalent line of walls designated against them and they will hold in position until the point the walls are all built and you can (in a 'retreating' manner) unbuild the scaffolding floors you don't wish to keep. If you use a bridge (to save on material, and maybe time if you aren't overloading your architect(s), or just to train your architect(s) up for fun/profit) then you need to set to build walls that will hang off the side of non-bridge places (initially the gap-adjacent wall-tops, usually, or the walls you already built there if you put a bridge across a wider gap after the fact) and then only as each hanging wall is completed should you command the next wall that hangs off of the hanging wall. Otherwise you risk building a wall that immediately becomes a cave-in hazard. (Which you can also do deliberately if you want to weaponise it in hitting something nasty below, but beware of the risks to the builder, and also any further tunnels/voids/unbuilt-areas immediately below the floor/ceiling tile being landed upon.)
ASCII diagrams available upon request. It's fairly simple to do, but somewhat less so to explain. At least the way
I try to explain it.