Siege!

It has become abundantly clear to you at this point that you are dealing with no ordinary enemy. As a result, orthodox methods of war are useless to you. You are going to have to seek unconventional methods in order to take this castle, for no amount of force can bring down this most demonic of enemies.
A council of priests, strategists and politicians determines that you are to be put into contact with the consultants to try to find a strategy to bring down the enemy. The first consultant is a murderer sentenced to death. He is adept at the art of killing but has been offered to be spared and forgiven of his sins if he can aid the church in destroying their enemies. The second is a heretic sentenced to be burned alive for committing vile acts against nature and God. He shall also be forgiving if his knowledge leads to the defeat of the enemy forces. The final consultant is a man shrouded in mystery. He claims to be a missionary here to help the Church defeat its enemies, but it is unknown to where he originates. When questioned he says he was sent by God, but a farmer claims that he arrived from the forbidden territories. All of these people are immoral and normally should not be dealt with, but they may provide the only means of defeating this great evil. As the saying goes, one must fight fire with fire.
The murder suggests to you that you should catapult the bodies of the dead along with dead live stock and other animals into the enemy fortress. You must be certain to let these bodies rot and decay before catapulting them. This way, when they strike, rotting foul meat ridden with disease shall spread disease within the fortress. This disease and plague will weaken the defenders so that they may be wiped out with ease. To do this you may need to sacrifice much live stock as well as use the bodies of soldiers who have fallen in combat. This tactic is sinful, but it may lead to the greater good of wiping out the infidels.
The heretic was condemned for attempting to access a profane magical art known as chemistry. Not much is known about this new evil magic except that it uses the element of the earth to forge unholy instruments of destruction. He claims to have knowledge of how to create a black powder which explodes when ignited or when struck with a great force. Some of this powder was seized when he was captured. Upon testing it you find that it can project objects as well as break through stone and even in large quantities metal. Perhaps this mans knowledge is not unholy but in fact a gift from God to fight the infidels. If you choose to release him he could help you produce massive quantities of this powder to knock town the walls of the fortress itself.
The third man states that he knows of the magic that the warriors from the uncharted territories used to enter the fortress, and he could lead you to the source of this magic. You could use it to enter the fortress yourself and slay the invaders. This vile magic exists to spite God, but perhaps after using it to stop the Forest People you will be able to destroy this source of magic, eliminating it forever. This would be a perilous journey though, entering the forbidden territories, but the Church is willing to make an exception to reclaim their temple.
These are desperate times and they call for desperate measures. What shall it be?