Like Dax said,
I usually hold up on the main building (the one with the corner wall at the top), though yeah, you are surrounded. It's not my favorite map, and the same can be said with Holdout. You'd think that Crows Nest would be a good place to stay at, but Skirmishers and Brutes just rush it and usually beat me to a bloody pulp. Overlook has a rock that I can usually stay on for most of the game, Outpost has the two AA guns, Courtyard with the area off to the side (where the sniper is in Campaign) and the Main Base when Skirmishers come into play, Beachhead on Roof, and on Corvette I just run up to the right side and stay in that room there on Waves 3+.