Since the game is supposed to take the player on a progressively difficult mode. I believe the human and elf armies are supposed to be the most difficult camps to defeat. So, why not make walls etc. to protect the camps so that you cannot destroy them without advancing your dungeon atleast a bit (maybe some monsters alone can get over walls without help). You could add some towers with archers in them to protect against some bats etc.
And if they have walls, why not include siege engines, bombs strapped onto goblins etc. to help blow up the walls or destroy troops. I am thinking of the lord of the rings movie concepts which would make the game really interesting and the gameplay even longer. This would help eliminate the unnecessary number of melee members in army (like suicide bombers).
