The above area must be watched carefully by the defenders. The attackers can easily grab the shotgun, pickup the overshield, and grav lift onto the main wall.
There are two other ways to breach the wall. The bridge and the gap:
The Bridge
The Gap
Normally, each round in an objective match lasts 3-4 minutes. So, in the last 30 seconds of each round, man-cannons will spawn, to assist the attacking team:

These will not spawn at the beginning. They will spawn at exactly 180 seconds into the round.
This map can be used for One Bomb, One Flag, Territories, Slayer, Team Slayer and VIP. This map was play tested, and the general feedback was that territories is the most fun.
Defend the Wall is on slot 1 of my fileshare. You can also download it here:
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