There was a Cliff DM map I played in. There is one base inside one cliff and one inside the other with a good 192 bricks of base room and only 64 inbetween the two cliff walls. However we were not allowed to even step foot on the other side of the bridge connecting them because going inside the other base was not allowed. There were even loving mazes inside the bases to make it more difficult to invade, but they served no purpose as we couldn't go inside. The reasoning was that appearantly it would make it too easy if we were to get inside the other base.
The point of a dm is to kill other people. If they are invading my base, I still am going to kill them. It's not like the second they step foot into my base I am incapable of shooting them or anything, why should such a handicap exist?