Legate's Camp

The camp is east of the Hoover Dam, apparently only accessible by a door inside the lower offices of the dam, and only entered with a key given during the last section of the final questline.

It appears that the only way to enter the camp is during the final leg of the main quest. Other than that, the camp remains sealed off from the rest of the map. It is surrounded by cliffs and mountains on all sides, inaccessible from the north (The Fort), the east (off the map), south (mountains), and west (mountains/cliffs/river/Hoover Dam).