Finally I have finished the undead fortress. I wanted a defensive fort or castle for my army. I didn't want to just build a fortress so I came up with the idea of a tower which was once an actual defence, but maybe a battle was fought there long ago and the remains of the tower became a graveyard, so a chapel was added to the tower on one side and a mausoleum on another side. The small bridge if mostly for effect with coffins bobbing up and down in the putrid pond.
On the far side is a smaller chapel in marble and a priest's dormitory. Around the base are a few gravestones to help the building fit in with the graveyard I built earlier.
So the fortress has a function to act as a centrepiece for the undead themed terrain. The fortress can be used by the freshly undead as a defensive bastion in their war on the living.