Silgrad Tower from the Ashes

A nice tutorial by Danatoth (TheNeuroSpasm).


My comments / resuming.

• Create a new faction for your NPC and add it to faction tab
  
• Set ownership (OWN) for the interior cell (faction)
  
• Set ownership of the door to the faction you made
  
• Create a key for your NPC and add it to the inventory
  
• Set locking of the door to the key you made
  
• Set the locking time through the AI pack e.g. when he/she sleeps

It's also possible to set the OWN of the cell and door to a NPC, e.g. check the WindhelmBelynHlaalusHouse interior.

I updated the OP.