Not to be content to settle on my first idea, I've had another that is more interesting than the basic "a wizard's tower"
The Blood God's Prison Ship
Every world has stories of those who came before. Sometimes, these stories are of cultures fallen just generations ago, to war or famine or strife. Sometimes, they are older – people who lived long ago and built monoliths, and eventually faded away. But sometimes, these stories are of people from elsewhere. Other planes. Other times. Other worlds. These Elsewhere People visit, leave impressions on local races with their vast power and knowledge, and then depart once more.
Usually, these Elsewhere People visit because they have a purpose. And in this case, that purpose was at once kind and careless. They remained long enough for stories to be written of them, placed a new star in the sky (called the Visitor’s Star), and then departed as mysteriously as they arrived. This was not a true star, but prison for a demigod. They could not allow them to live freely, for fear of the destruction they would cause. They could not allow them to die, or else they could ascend to true godhood. And they could not imprison them on their home world for fear of someone setting them free. So, they created a prison, aboard a satellite, to around this distant world, for all time.
But the longer the scale, the less you can plan for. It has been hundreds of years now, and the prison’s systems was struck by an errant comet, knocking it out of orbit and sending it eventually down to the surface of the world. It will break up in orbit, and scatter across the surface of the world.
The ship had many “ward” modules, and was able to rearrange its shape to a limited extent. Each ward module had a combined purpose of keeping the blood god contained if it were to escape, keeping anyone else out who wanted to enter, and housing part of the station’s systems, and part of the key to the blood god’s cell, so that to truly escape . As such, even if one module is damaged, the station would still function… except that some modules are more important than others. There was only one control module. And there was only one orbit module, and it was this which was struck, exploded, and caused the station’s fall.