The legend is that if the ravens leave the tower something bad will happen. I think that these birds can all fly away if they want, so I don't know what keeps them there. Maybe their wings have been clipped so that they can only fly a little bit. They are ringed, so they know which bird is which.
Some guardsman chappies. Who knows if they really do any guarding, though. Lob a grenade in there and find out.
