The alternative is one tile of floor outward, which makes use of the fact that they can't hold onto the air above the floor. See the wiki diagrams here.
I stand corrected. For walls, I've thought 1 tile will do, but only if it is a floor it seems. Floor does not stop crossbowmen from jumping out though.
I am building floor around my fortress. I wish I could build paved roads, but they are finicky in many aspects after construction. Floors are way faster to deconstruct and reconstruct, don't preserve stuff under and dwarves do not blink on them, so you can't see if they are carrying something or not like on paved roads.
Tbh, I always build a bunker. Not a castle. I am even so kind to provide up/down stairs to its top. Just in case, if something climbs up there, then it will have no buggy issue climbing it asap down too.
I could build archery top floor with fortifications instead of walls. Floor, which opens only for sieges. Stairs to inside of the bunker covered with a bridge of course. However shooting from height mechanics is bugged currently. Height is not an advantage adding to precision and length of shots. Height just merely adds to shooting distance. This makes shooting from height useless and less accurate.
Better is to build on floor level of surface bunkers with underground access, which have double fortifications on all sides for walls with 3rd wall being bridge, which you can close and open with lever. You need to train your military to legendary crossdwarves to use it. Then just put 4 such smaller bunkers on corners of your main surface "mountainhome" bunker. It should be as safe as hall ending with 120 cage traps narrow passage. Quantum dump pit for bodies in your favorite mass pit with few special very happy dwarves (eat quarry bushes or so, drink dwarven alcohol - ones) enabled to carry corpses. I would recommend only having coffins ready for burials of your own dwarves or some material set on side for making memory tables. Training for legendary marksdwarves will be necessary and annoying with all those buggy training/combat ammo switches, but micro-manageable.