Two problems with the retracting bridge o' death:
1) It won't work on megabeasts, since retractable bridges will not operate while a megabeast is on them (per the wiki).
2) It won't work on standard orcs, since they have TRAPAVOID and, to my knowledge, will not trigger pressure plates.
One way I can think for a pressure plate to work would be if corpses can't trigger pressure plates. (I don't know one way or the other.) If they can, you might be able to wrangle a creature onto the spot where your pressure plate is, and as long as the creature is on that spot and alive the trap will be in the "off" position. When the creature moves or is killed, the trap will flip to the "on" position. This way an invading army of orcs that slaughters a bait kitten on the way in will end up triggering the trap. However there are a lot of ifs in this concept, and you also have to account for situations where the creature dies of old age. Resetting the trap is also not a simple matter.
Another option is to build a hallway with a low covering of water like 2/7, seal it off with doors, and make that the most direct entrance to your fort. Anyone who opens the doors would spill water into the area outside the hallway, which you can detect with a pressure plate. However this is somewhat bad news for traders and guild reps who may path through there as well, to say nothing of your own dwarves.