Castle Karstaag Ruins is a dungeon in Skyrim: Dragonborn
Northwestern shore of Solstheim.
When you emerge here from the Castle Karstaag Cavern, there are more Rieklings waiting for you. This does include one Mounted Riekling on a Bristleback Boar. They are likely to have a Castle Karstaag Key that you will be able to claim and open any remaining doors.
Defeat the Riekling and turn to the North. There is a pair of Riekling Scouts there.
You will find a throne there. If you have Karstaag's Skull, then you can bring it to the throne and place it there.