> you would need to buy a mason (so that you can make your towers stronger/repair them),

Yeah, that's the part I don't quite like about the castle/camp difference: It might make it more laborious to make actual castle. On the other hand: Camps should still have their place, and what place would be better than such throw-away bases? Once you have a few cannons throwing flints at you, it will probably be lost anyway.

> a forge (for a shield), an archery/carpentry for the bow and a chemical lab for the flints.

Well, you would most likely try to settle for one kind of attack and only buy the infrastructure for that. I am not afraid to admit that this is inspired by Starcraft: Lock the player into certain types of attack, so the enemy can in turn make strategic decisions how to counter them.

> Where would you sell stuff?

At the flag. Just bring the lorry near it, ka-ching, all valuables sold.