PS : I find it too painful to make a aboveground of decent size. The corners especially. brrrr !
Build corners first, where trying to wall up against an edge (access exists from both neo-corner wall spaces, and needs one kep clear, but both is useful - when corner is built, both neo-corner blocks can be filled in, and you may have already built the rest of the non-corner walls by that point)
Build
an internal corner last when sealing off an inner space you don't want to wall yourself into (it forces the last building action to occur not-in-the-sealed-off-area - also a useful technique for underground sealing off of caverns without messing about putting suspended wall jobs to force the issue, if you find the right spot to put your barrier up)
When putting walls atop walls, start at the point(s) furthest away from the climbing-up-onto-the-walls access (if you aren't, or aren't yet, building walkways alongside the walls) regardless of whether that's a corner, then as soon as it is (or they are, as you can do two next to each other) built define the next wall, regardless of corner or edge, round each way towards the point of access (it takes micromanaging, but it is obvious what you can build without blocking further building).
Though digging out space is still easier,
most of the time, than enclosing space aboveground. Fact of life. So much so that the extra niggling difficulties
perhaps qualify a suitably practical aboveground construction site to be the epitome of dwarfiness in the field of engineering! (If not in the field of fields.)
(I advise no moat filling
at all, at least of water, but that's partly because I design mine to make it possible to retrieve anything dead, and anything possessed by that dead thing (including 'decorative steel bolts through the skull'), should it find itself in tnat state and ditched (or ditched and then given that state), friendly and unfrkendly alike. And partly because I spend time digging a deep ditch and to fill it back up a z-layer or two tends to undo the effect of any drop to the dug-to degree, even if drowning/etc is still an option. IMO, though, YMMV.)