To beat all the levels with forts all you have to do is shoot all the raptors when they are attacking the fort. A secret is that you can reload and repair the windows all at the same time. Another thing make sure to constantly check the other fences killing or wounding any raptors you see. But if you see the raptors entering a window a distance away kill it and quickly run over to the window and repair.
Also make sure that you get more ammo from the ammo box and use your melee attacks is necessary.