Siege!
You have the troops begin setting up camp around the fortress. The forest people need to eat just like any other living creature. Even demons require the sustenance of souls and flesh. You have time to wait them out and constant supply lines to keep yourselves fed. When they starve you will be able to simply march in and dispense with the weak survivors with ease. After two days of surrounding the fortress, a theist expert arrives to consult with you about the siege at hand. In your private tent he tells you that the siege will not work.
He elaborates, explaining that two artifacts of great divine power were stored within the fortress that will provide them with an infinite amount of resources. The first, known as the Holy Breadbasket is a basket which can produce enough food to feed whoever is hungry near it. Its magical property makes it a source of unlimited food, so the forest people within cannot be starved. The second artifact is the crusaders forge which produces weapons and supplies such as arrows similarly to how the breadbasket produces food. With these two items in their control, the invaders can last within the fortress walls indefinitely. Essentially, you are going to have to take the castle by force.
He elaborates, explaining that two artifacts of great divine power were stored within the fortress that will provide them with an infinite amount of resources. The first, known as the Holy Breadbasket is a basket which can produce enough food to feed whoever is hungry near it. Its magical property makes it a source of unlimited food, so the forest people within cannot be starved. The second artifact is the crusaders forge which produces weapons and supplies such as arrows similarly to how the breadbasket produces food. With these two items in their control, the invaders can last within the fortress walls indefinitely. Essentially, you are going to have to take the castle by force.