hmm it would nerf forts but might be counter-intuitive to a player. my intention was that the tiles are temporarily yours, and works just like any other of your tiles, but when you lose a fort you lose tiles too. so they can be shown as regular tiles of your ownership, only city screen needs clarification as its a screen where you buy land. so they should to be marked somehow there. although making them unworkable would be ok too only you but not other civs. no one can grow to already owned tiles, if fort is destroyed or captured they return to an original owner. only if original owner city gets destroyed, those tiles become nobody's and can be accquired by other civs. you can think of fort surroundings as of a second layer of ownership (temporary ownership, occupation). the blue player can grow to tiles 3 and 4, can not grow to tiles 1 and 2 (but can work on them) the red player can not grow to tiles 1, 2, 3 and 4. the red player will get tiles 1 and 2 back, if fort is destroyed. i think that could be an interesting effect but as you mentioned potentially exploitable. or maybe not really exploitable but just unfun, as player will be forced to build them everywhere to increase his borders growth. what will render this feature redundant.