CanuckSoldier
Emperor
In case it's not clear from what others have said: Build a fort on a resource makes that resource available to the trade network, but it doesn't give you the local production bonuses that a mine (or other improvement) would give for working that resource. So you only want to use a fort on a resource outside the city radius; use the normal improvement inside the city radius when you're going to be working that tile.
You mean outside the "fat square", outside your "city radius" aka your borders give you nothing but a defensive bonus on a tile.
CS