Amendment to rule #3: Choosing colors and castles.

The main idea is how to combine a Castle and a Town?

No, it's actually without a Castle.
The generator randomly assigns them. Some will get better ones, some worse. And the one with the better Castle (subjectively) offers a certain amount to play with the Castle that the other player has. The opponent can agree and accept the amount, or disagree and offer more to switch Castles.