Before Starswirl passed,

You are assuming he is dead. The show has never actually mentioned him dying so honestly a lot of it is up in the air still.

 

I am hoping we will get an episode about the the origins of Celestia and Luna, and we may find out more about the castle then.... for the moment though, it was just the original home of Celestia and Luna until Luna was banished, upon which Celestia could not longer remain in the castle... the memories were just too painful.

 

I do think the castle will play an important role in the future though... or at least I hope so, since I have a couple of theories on what could be going on, or at the very least where they might be planning to take it... although only time will tell.