In that small map the marines and survivers for the most part are just defending the radio from the zombies before and after the whole kill Fred thing. I was expecting after a hoard of zombies were up on the gate that it would break and thus make it easier for the zombies to get to you and the radio.

For that map you might want to make the respawn system different such as survivers have a limited amount of spawns... say 20 for the map. (number can be adjusted.) That way the survivers really try to keep the zombie hoard from getting too close. At the moment the only way for the zombies to get you is to go thru the middle opening or for a player zombie to jump the fence.

I say make the fences breakable after so many hits from sombies.. say 50 (number can be adjusted) and near the closing of the map make the zombies in more abundance spawning closer and maybe make them tougher to give that "grave" feeling of over whelming odds just as the chopper pics you up.