You cannot turn the town in a fort. But you can try to win a fort in a battle, if you have enough manpower. There are 3 forts in each county: 1 small, 1 medium and 1 large. If they are gray, you can start building it. If they are all red, it means all of them are already taken by other towns and you will need to hit the attack button and show up with lots of players to try to win the battle and take the fort.