"Hij is in het kasteel."
Translation:He is in the castle.
Haha, not bad! One small mistake: "maar" is a coordinating conjunction, not subordinating (just like "en", "of" and "want"). That means that the second clause has a normal word order and the verb is not moved to the back. Correct would be:
Het spijt me, Mario, maar de prinses is in een ander kasteel!