[Taqwus]

There are a few Earth spells relating to sieges. One of 'em, if memory serves, does huge amounts of damage to the castle and the resulting collapse can hurt the troops inside, too. Another note -- while assassins can't get inside the castle during the siege, assorted assasination spells can like Seeking Arrow and Earth Attack.
But yeah, bringing more troops will help. They don't even have to be living, thinking troops.