With a bit more time, I thought I might demonstrate some other "tower-building" techniques for you.
This time these are plan-views. # = walls, + = floors (coloured when newly added), X = stairways (up/down, here, but could be just up or just down when bottom-level or top-level items...)
At the ground level (; = natural, grassy floor),
imagine you've enclosed a 5x5 space, except for a doorway, and put a stairwell in the centre...
;;;;;;;;;;;
;;;;;;;;;;;
;;#######;;
;;#;;;;;#;;
;;#;;;;;#;;
;;#;;X;;#;;
;;#;;;;;#;;
;;#;;;;;#;;
;;###;###;;
;;;;;;;;;;;
;;;;;;;;;;;
On the level above you'll start with the following (. = open space)
...........
...........
..+++++++.. <- these floors are tops of the walls
..+.....+..
..+.....+..
..+..X..+.. <- build a corresponding stairway (accesses from immediately below)
..+.....+..
..+.....+..
..+++.+++.. <- notice the gap. No wall beneath here
...........
...........
...........
...........
..+++++++..
..+++++++.. You probably want to define all of these floors,
..+++++++.. but you can't do them all at once in a 5x5 rectangle
..+++X+++..
..+++++++..
..+++++++..
..+++.+++.. (also, for this level, we'll leave this gap unfloored)
...........
...........
...........
...........
..+++++++..
..+11122+.. The "quickest" way to define all those floors is
..+11122+.. as the numbered groups... Four 2x3 floors (two being 3x2s)
..+44X22+..
..+44333+..
..+44333+.. (There are other ways... two 5x2s with two 2x1s... work it out...)
..+++.+++..
...........
...........
...........
...........
..#+++++#.. Now that you've got edge-access, build the corner walls first!
..+++++++..
..#+++++#..
..#++X++#..
..#+++++#..
..#+++++#..
..#++.++#.. Or all but one along the edge, so there's a corner-adjacent
........... floor to build the corner while the corner needs completing.
...........
...........
...........
..#######.. Fill in the gaps
..#+++++#..
..#+++++#..
..#++X++#..
..#+++++#..
..#+++++#..
..#######.. The wall over the gap is no problem.
...........
...........
Now we have another level (Z+2) that has the same wall-tops as Z+1 was before we added these walls.
...........
...........
..+++++++..
..+.....+..
..+.....+..
..+..X..+.. Another stairwell access
..+.....+..
..+.....+..
..+++++++.. <- No gap, this time!
...........
...........
...........
...........
..+++++++..
..+..+..+..
..+..+..+..
..+++X+++.. An optional start (get the other floors in later) is to just send
..+..+..+.. floors out to the wall-tops...
..+..+..+.. (You could have done this on the first level.)
..+++++++.. <- No gap, this time!
...........
...........
...........
...........
..#+++++#.. Corners first!
..+..+..+..
..+..+..+..
..+++X+++..
..+..+..+..
..+..+..#..
..##++++#.. Or corner-plus-one-edge first
........... (each wall being built has a place to build from).
...........
...........
...........
..##+++##.. When each new wall is built, you set walls
..#..+..#.. on the adjacent floors that you were
..+..+..+.. building from...
..+++X+++..
..+..+..#..
..#..+..#..
..###++##..
...........
...........
...........
...........
..###+###..
..#..+..#.. Keep on 'building in' to the accesss..
..#..+..#..
..+++X+++.. But don't build on the final wall-top
..#..+..#.. until you've got both sides set!
..#..+..#..
..###+###..
...........
...........
...........
...........
..#######..
..#..+..#.. (Sometimes you get the dwarf building the
..#..+..#.. wall in the final gap get interrupted by
..#++X++#.. "creature occupying site"... him or herself.
..#..+..#.. I don't know why this is. Cancel the suspended
..#..+..#.. wall and redo after the original wall material
..#######.. is cleared (used elsewhere, or taken to a stockpile)
........... and it should eventually work out.)
...........
... insert remaining floors at leisure. (Not so useful in the above.
Might be something you want to delay with much larger tower floor-plans though!)
And what if you want to extend an overhang?
What if the corners are not on top of wall-tops, but one tile further out?
(Also applies if you've extended floors over the edge, and are building
the walls one tile beyond them...)
...........
...........
..+++++++..
..+..+..+.. You can actually do with less floor to edges
..+..+..+.. (as you could have done in the previous example
..+..X..+.. although instead of all corners first you'd
..+.....+.. have had to have started with a wall directly
..+.....+.. opposite and the pairs of walls adjacent
..+++++++.. until meeting back at the access...)
...........
...........
...........
..+........
..++++++++.
..+..+..+..
..+..+..+.. Set up some temporary floors,
..+..X..+.. semi-adjacent to the corners you intend to build.
..+.....+.. (Will remain coloured.)
..+.....+..
.++++++++..
........+..
...........
...........
.#+.....##.
.#++++++++.
..+..+..+..
..+..+..+.. Now set to be built both the corner-walls
..+..X..+.. ('floating', but supported by the temporary floor)
..+.....+.. and the wall for the other corner-adjacent tile
..+.....+.. (which is supported by the wall-top of the corner below).
.++++++++#.
.##.....+#.
...........
...........
.#......##.
.#+++++++..
..+..+..+..
..+..+..+.. Now you can deconstruct (designate to remove construction)
..+..X..+.. the temporary floor. The corner wall is held
..+.....+.. up by the first adjacent-to-corner wall.
..+.....+..
..+++++++#.
.##......#.
...........
...........
.#########.
.#+++++++#.
.#+..+..+#.
.#+..+..+#. Every remaining edge-wall can be set to be built,
.#+..X..+#. now, the workers safely standing on the wall-tops
.#+.....+#. with no access problems (even with just that one
.#+.....+#. line of floor leading from the stairwell...)
.#+++++++#.
.#########.
...........
(Actually, I'd probably set all the walls except the ones the temporary floors
are in the way of simultaneously with my setting the temporary floors for removal.
Add the walls that these floors obstruct as each floor is deconstructed.)
((Also, deconstructed floors used to have their materials fall to the level below, when
finally removed. These days (probably to avoid falling damage to those below) the material
is placed on the bit of adjacent floor from which the unbuilding happened. Useful. Also I like to
use a different-coloured stone (a rarer one, like green olivine) for temporary structures, so I can
see at a glance what I've still to remove. With this new 'recovery' movement, the temporary
olivine stone (or whatever it is) is closer to the next location I might want to use it...))
Fill in the remaining floors at leisure...
For a more "major" overhang extension, and one where you've got
door-gaps to fill, try something like the following...
...........
...........
..+++++++..
..+.....+..
..+.....+..
..+..X..+..
..+..+..+..
..+..+..+..
..+++++++.. Sending as spur of planned floors out beyond the gap.
.....+..... (If you're not covering a gap, here, you can cancel
.....+..... the building of this bit of floor, but here I'm assuming
.....+..... there is one, as in the Z=0 wall opening, whether filled
.....+..... by door or floodgate or raising bridge.)
.....+.....
...........
...........
...........
...........
...........
..+++++++..
..+.....+.. (You don't actually have to wait until all the
..+.....+.. floors have been built before setting the rest to be
..+..X..+.. built, but remember that it's a Last In/First Out
..+..+..+.. job queue, so that except for any floors currently
..+..+..+.. down as being started (i.e. not waiting), the ones you
..+++++++.. leave still to do will languish behind all the newer
.....+..... floor-building jobs you add. You can flourish this in many
.....+..... different ways, for fun and profit. Or you can end up having
.....+..... to wait (or cancel and redo to add a floor you need now
.....+..... to bring it to the front of the queue.)
..+++++++.. Extending some floors, including more
........... temporary ones...
...........
...........
...........
...........
..+++#+++.. Only one wall can be safely built, somewhere opposite
..+.....+.. the place that has walking access. (Maybe two adjacent ones,
..+.....+.. but you tend to get dwarves trying to work on the spot each other
..+..X..+.. is standing on...)
..+..+..+..
..+..+..+..
..+++++++..
.....+.....
.....+.....
.....+.....
.....+.....
..+++++++..
..##...##.. Two 'hanging' walls, one relying on the temporary floor.
...........
...........
...........
...........
..++###++.. Extend the walls around
..+.....+..
..+.....+..
..+..X..+..
..+..+..+..
..+..+..+..
..+++++++..
...+.+.+... More floors (you might have set these up first
...+.+.+... instead of the 'hanging spur', actually)
...+.+.+...
...+.+.+...
...+++++... Temporary floors removed
..#######.. End wall can be completed, in this setup...
...........
...........
Skip a few steps and you have...
...........
...........
..#######..
..#.....#..
..#.....#..
..#..X..#..
..#..+..#..
..#..+..#..
..#+++++#.. Nearly closed everything off.
..#+.+.+#..
..#+.+.+#..
..#+.+.+#..
..#+.+.+#..
..#+++++#..
..#######..
...........
...........
...........
...........
..#######..
..#.....#..
..#.....#..
..#..X..#..
..#..+..#..
..#..+..#..
..##+++##..
..#+.+.+#..
..#+.+.+#..
..#+.+.+#..
..#+.+.+#..
..#+++++#..
..#######..
...........
...........
...........
...........
..#######..
..#.....#..
..#.....#..
..#..X..#..
..#..+..#..
..#..+..#..
..###+###.. What's left is a gap for a door...
..#+.+.+#.. And you can fill the rest of the floors in at leisure,
..#+.+.+#.. and start anew on the level above...
..#+.+.+#..
..#+.+.+#..
..#+++++#..
..#######..
...........
...........
...........
...........
..+++++++..
..+.....+..
..+.....+..
..+..X..+..
..+.....+..
..+.....+..
..+++.+++.. The level above (wall-top floors!).
..+.....+.. Note the gap. Again...
..+.....+.. But by now you know how to work with that!
..+.....+.. (Get around it and/or use it, that is.)
..+.....+..
..+.....+..
..+++++++..
...........
...........
Oh, and Larix is right. One other thing, though, is that constructed fortifications don't have wall-tops. So if you're building fortifications on one level you don't have automatic 'floors' on the level above and you have to design as if you have "overhanging" or floating walls, like that final "upper level extension" is made.
However, constructed walls (with wall-top floors) can be carved into fortification (still with wall-top floors). Needs stonecarvers as well as builders, but depending on what you are building it might be easier/quicker to do it with a mixed workforce.