User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10901795-20130714155616/@comment-3146930-20130714160300

The borders are most likely a perfect sphere, meaning you couldn't even dig under the camp, or Kronos would have just tried that instead of going through the Labyrinth. With Rachel, Percy was thinking the borders would keep her out like a force field. Annabeth was worried that Peleus would eat her. She was either invited into the camp by Argus or fate decreed that she could enter. I don't think they ever say she flew over the defenses or, once again, Kronos would have done that.