"work In Progress Castle"

Before the Ocean Update, there seemed to be a some kind of work in progress castle in the "ocean" along with Mage Island that you can also see from old map pictures. So what was that castle? Does anyone here know what it possibly could have been used for and the possible reason for its removal?

 

Go look at Bob's fighting area and you'll find out something interesting

 

I can see a castle in the background. But why is it there?

 

I assume it’s just a set piece. Nothing about Bob’s Reincarnation makes much sense :-/

 

Bump.