Actually that's an excellent idea. I think I'll do that.

There's also an ominous cave just behind the keep. I think that will be a last refuge - a little like the caves at Helm's Deep, but that might be a bit too derivative. Hmmm, I'll have to think about those.

Another thought was that the attacking force had a plan to bring down the mountainside on the keep by getting into old tunnels and setting alchemical explosives.

So many options.... really need to make up my mind.