Following my experiments with floating traps, I have designed a Dwarven Force Field, for protecting large outdoor areas against flying enemies (creatures or avian-mounted invaders).
Build a rectangular grid of outdoor stairways, as high as the volume that you wish to defend. Space the stairway columns no more than twenty tiles apart. At every level, build a one-by-X (eg: one-by-ten) retractible bridge, extended towards the next stairway in the chain. Using these retractible bridges as catwalks, build a stone wall completely around the perimeter of your stairway grid, adjacent to the catwalks and as high as your stairway columns minus one level... in other words, the very top of your stairway columns should allow you to walk on the top of your finished wall.
Now cover the top of the wall with traps of your choice... probably cage traps or weapon traps. When all the traps are in place, demolish the top level of the wall. The traps will remain floating unsupported in mid-air. Now (with the top level of the wall removed), cover the new top of the wall... one level lower than before... with traps. Then demolish that level of the wall. Cover the new top of the wall with traps. Repeat until you are back at ground level. Finish off by trapping the place at ground level where the wall used to stand, then retract all the bridges.
You now have a three-dimensional force-field, floating unsupported in mid-air, protecting the enclosed volume of outdoor space.