Can you Carve Fortifications into raised bridges?
I am guarding my fort's entrance with a ballista, and I am using a drawbridge to prevent goblins from getting into the fort. But I don't know how to keep the bridge raised, and allow the ballista to fire at the goblins at the same time.
The simple answer to the question is: No, You cannot carve fortifications directly into bridges.
You can, however, solve your problem by altering the layout of your entrance to have the same effect.
...╔═╗| ...║+║| ...║+║| --┐║+║| ??|║+║┼ ??|║+║┼ ??|╚═╝┼ ??└FFF┘
Here, the bridge retracts up to the right, still blocking access to the doors for flying creatures. Yet, it allows your ballista (positioned behind FFF) to fire at things in its path.
You can make the bridge longer to allow for a greater range.
The design can then be further improved: build a second bridge over the
...locations, also have it retract to the right, to trap invaders on your bridge.
At this point, you might just decide to see what happens if you lower or raise the main bridge while there are invaders stuck in the trap and discover the https://dwarffortresswiki.org/index.php/DF2014:Dwarven_atom_smasher . Which may or may not be considered an exploit. For what it's worth, you do lose all the loot, so having the ballista can be useful if your goblin(s) happen to be wearing something very valuable to you.