This fort uses ramps as a tunnel. So you're able to walk over the fort easily, while they have to crawl all the way along the wall to get in. Also, I put extentions of the ramp above the wall like a barbed wire fence, facing down. That way you can pretty much hide behind your ramps on any part of the wall and choose where you want to go.

this shows the area you crawl in from the exit inside.

This shows the ramps you are able to walk up. Notice how if you spaced out the ramps, you could easily shoot people crawling in their heads from point blank range, while you have cover.

Another pic of the exit on the right.

This is an outside view.

This is the entrance on the right from the outside. You don't really need to make another tunnel outside, but I finished early and slightly sloppy too.

The entrance to the ramps. You should probably double layer the first few ramps.

And the ramp underside from the entrance.