Ok first off, this took way too long(3 1/2 hours). But I was bored.

The entrance is in the back and is an incredibly hard crawl and jump space. Also there is a platform next to it so defenders can shoot anyone going through. Its protected from enemy fire.

Looking out.

The flag is above everything. The defense would have to be stupid to let someone take it.
Like I said this would be impossible to build in a normal match unless someone made a clan and divided up the labor and practiced forever to get it in time.