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

in the Naga city, I took the same model for the map that was in the city
if in Heroes 7 the model on the map changes with the upgrade of the capitol, then you can put the same model instead of the capitol in the castle; the other buildings, however, will remain unchanged.

Then you can do something. On the map, we place a model from Heroes 7. In the castle, we also place it, but larger, like a capitol, town hall, etc. You can create battle maps like in Heroes 7, such as an attack on the center - on the town hall (capitol, magistrate).