Hmmm.
It might be difficult to annihilate a 5,000 strong force.
If it's me, though, and I come at the entire force with the intention of killing them...
But even just being there, the other side should know about it.
Information that a King and Emperor class magician are present should leak out somehow.
It would be the height of foolishness to just plunge into such a well defended fortress.