RTF is a global modification that replaces all towns in the game.

Regarding the new castle, it's possible, but the question is where to get the models for the castle's interior.

In the Naga city, I used the same model for the map that was used in the city itself. If in Heroes 7 the model on the map changes with the upgrade of the capitol, then the same model can be used instead of the capitol in the castle; however, the other buildings will remain unchanged.