Right, well, this is where it gets, tricky. imho (in my honest opinion), you should make the two teams spawn on opposite ends of the map, and they have to rush to get to the fort.
There is a way to swap the teams around, but I honestly think you should work on your mapping first.
But who am I to get in teh way of a total nub, to swap teams you need to have a load of trigger_teleports under the players, they drop through, you then swap which destinations teh teleports go through, it;s all very complicate dplease don't even try it.
Thank you.