Here are some pics of the basic layout of the fort me and a friend of mine make on sf_field trip. After lots and lots of rounds we have tweaked it to something like this, and no one has been able to beat it yet. Each time we make it the basic outer wall and entrance stays the same, but we change the details and the other structures we build all the time.
Front Wall:
Nade Tower and shooting place thing
Main Entrance Tunnel
One of the forts it faced:
The entire fort main wall is overlapped so you have a double wall with 25% less pieces and the wall around the flag is super thick. The nade tower on the right lets you easily nade the other base without sprint throwing. Thats basically it. Ohh yea, and the fight time was 10 minutes what we did to the other base.