@Kaarme That assumes he'll be facing adventurers with no ranged options, zero ability to deal with traps, no idea how to skirmish, no equivalent to magic missile, etc.
@Wordsmith Sure, no matter what sort of defences and troops he had, one could always create a list of enemies able to overcome them. But currently he's in a situation with a tiny dungeon empty of all defences and not too much resources to correct the situation.