Guard dogs are great if you've got enough to spare killing them wantonly for the purposes of security. That's up their with in idea space with painting your fence with the blood of baby turtles. And, while I fully endorse the idea of forcing the nasties through labyrinthine corridors to get to your fortress, ideally through hordes of nasty traps, that seems more like an anti-siege idea. And the problem with cage traps and caged animals is that you have to refill both. Since war dogs are no longer the gods of war that they used to be it takes half a dozen to kill even a moderately armored and trained invader, and I can think of better things to do with metal and wood than make the worlds largest goblin zoo.
I get that there are ways to work around the fact that there is literally no early warning (yes, literally, because even the dogs and cages have to bump into the goblins to see them), but, honestly, settlements in any period that would have had walls would have had people watching out from on top of them precisely so that invaders wouldn't sneak past them. And it's not like it'd be removing the difficulty: You'd still have to deal with them, they wouldn't just vanish.
And additionally: Can you name one scenario in which it would make sense for a group of heavily armed, semi-self-aware, evil humanoids would be able to sneak past people within whispering distance of them without arousing an alarm? For that matter, can you name a scenario where highly paranoid, highly militant, massive fortress building humanoids wouldn't bother to look out for the nasties they know are out there?
-D.B.