First of all, I've never seen the chains glow red. They might with high perception, but the expert perception teacher is in Thjorgard, I believe, and anyway, you don't need more than perception level 1 to break into the keep in the first place.

You will need a ranged weapon skill and equipped ranged weapon.

Go in to the room where the lever is that you opened the portcullis with. Look up on the ceiling; there are chains there (2), but not red. Just shoot at them until they break; the drawbridge should fall. If they're not breaking, something is wrong; it doesn't take long - I just finished doing it with all bow level 1 characters.