If a goblin snatcher or thief, or whatever manages to escape while they saw your dwarves construct a trap all future attacks can avoid the traps next time, or something along those lines. I am not sure how it exactly works but you should be careful when constructing traps to avoid sight by hidden enemies.
You can make a bottomless pit/chasm by channelling the very lowest layer, so you don't actually need a chasm in the region but only a water source for your experiment.
Hmm.. well things wouldn't be able to fall and be stuck inside the bridge really, so it sort of behaves like a wall, but dwarves can't exactly walk onto the raised parts either. Someone would need to drop a dwarf on a raised bridge to see what happens, in my opinion the dwarf would be just stuck there.
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::shrug:: I guess I didn't need this space after all!