The empire is placed on a mountain near the castle. The undead will try to overwhelm you and will send all their units to the castle. This will cause heavy losses on both sides, but even artillery and arrows will not achieve huge decimation of the enemy's units.
Therefore it is recommended to defend Bogenhafen, because taking back the castle is much easier.
After finishing the mission succsessfully you will be given a potion of strenght by the tower commandant.