Obviously Starlight. Twilight even said that she’s gonna be in charge of the castle and the school once Twilight steps into the role of ruling Equestria.

You know that monarchy always have more than one castle/palace right?

@Mori  
Just noticed. My bad.

@Background Pony #D61D  
The School of Friendship is right next to the castle in this picture.

Most likely Starlight and probably Trixie and Sunburst moved in since they became permanent staff of the school, they probably still have their friends that stayed on Ponyville come over for parties and the sort. There is also the possibility that Smolder and Ocellus live in there since they are teaching at the School as well.

@Background Pony #D61D  
Most cities have expanded due to the more open foreign policy. Ponyville must have exploded in growth.

Who lives in that castle?  
No one lives there.  
Then who is your lord?  
We don’t have a lord. We’re an anarcho-syndicalist commune.

Wasn’t that Castle located next to the lake and the School of Friendship?

Twilight had to stay there and live with her friends, just Hasbro has no brains.

Ghosts. It’s a “haunted house/castle” tourist trap.

In the Season 9 premiere, Twilight expressed plans to have Starlight watch over the castle in addition to wanting her to take over the reigns of the School of Friendship. Also very likely Sunburst moved in when he moved to Ponyville to become the school’s vice principal.

I’m sure it’s Starlight.

Starlight, probably Sunburst, Trixie’s most likely got a room.
 
And I’m sure they rent it out.
 
They’d be silly not to.

Well Starlight did already live there

I assume Starlight or a student 6.

Starlight