Absolutely, you can build a drawbridge on flat ground. Be aware that you won't be able to channel the ground out afterwards without deconstructing the bridge, not even if it's raised. But if all you want is a controllable wall, it'll work fine.
And you can't designate a stockpile on/under them either. Is it possible to designate an activity zone on/under a bridge tile, or if not, what's the trick of the Dwarven Atom Smasher?
Other question: is there a way to permanently or temporarily make a pet, humanoid, war-trained creature into a something that can be conscripted into my military, and permanently assigned equipment? I've read about Cacame, Elf King of Dwarves, and back then it was possible to do with Dwarf Companion... however, AFAIK DC isn't supported by the current game version.
The aim is to take my modded tigermen (pet, with innate weapons skills, trainable and already war-trained, [EQUIPS] so they come wearing large clothes but no armor, and equipping no weapon despite [HABIT_USE_ANY_MELEE_WEAPON]... shouldn't that have them make a beeline to the nearest weapons stockpile? IIRC it works fine with minotaurs where it's copied from...) actually pick up weapons and if possible shields. Bonus if they can stay conscripted into squads after being switched back from dwarf to war tigermen.