Just as the title details, after you place ANY siege and it decays it'll remain on the map and take up a siege placement for the keep. You can't even place siege around the ghosted ones.
https://www.youtube.com/watch?v=JySofS-tBJQ
https://www.youtube.com/watch?v=LJlAEV7xTbc
https://www.youtube.com/watch?v=JySofS-tBJQ
https://www.youtube.com/watch?v=LJlAEV7xTbc