All of the suggestions have been good ones. Undead and mercenaries are a good way to go, especially since they should be prepared for future attacks. They should also have scouts out on patrol in the area around the fortification at this point. The cult should probably also take the time to reinforce their fortifications.

If this is just a hideout for a group of outlaws (religious or not), they should probably leave by now. They've been discovered and criminals don't like clashing with the authorities. If they're not outlaws, they should be able to call on the authorities for aid and can probably describe their attackers to the authorities.