The Legate's Camp is in the Mojave Wasteland, located east of Hoover Dam. It is surrounded by cliffs and mountains on all sides and is inaccessible for the majority of the game. The only way to enter the camp without using exploits is during the final leg of the main quest. Other than that, the camp remains inaccessible.
At the entrance of the camp there are a few Praetorian guards. One is standing directly in front of the hill and two are in the small encampment to the left. On the right side of the entrance to the camp there is a cage with Legion mongrels. The cage door can be sabotaged so it won't open, but the dogs inside can still be attacked.
Up the hill, you will see Legate Lanius. Approaching him will initiate conversation. On top of the hill stands the Legate's War Tent. As the tent is locked from the inside, (which is quite odd considering it's a tent), there is no normal means of entry. Inside the tent, during the Veni, Vidi, Vici quest, you will find him with a Praetorian Guard on the right and a slave sitting on the left. The Legate is torturing and mutilating NCR prisoners, as you can see from the bodies on the table and floor.
If the user has enough complimentary vouchers they can access the camp before the final missions. However the camp has up to 15 well armed Legion soldiers when the player arrives. (Confirmed on Xbox)
The Quickest way listed is to go to the NCR side of the Hoover Dam checkpoint, to your right, (on the outside) there will be sandbags next to one of the long canopy things, use a complimentary voucher (armor jump, or ammunition stairs, etc.) to get on top of the canopy. Then, go over the checkpoint, Using one of the earlier methods of climbing, head towards the Legate's Camp, and, when you reach the giant landslide area, GO RIGHT toward's the other wall (the one without the centurion guarding it). There will be an invisible wall, BUT go towards the cliff, DO NOT FALL OFF! then follow it you will reach an area with a smaller cliff to your left, follow it, until the road to the camp is visible. head north to the mountains, at thuis point you'll have to jump off a few cliffs, so Med-X and stimpaks are advised. sooner or later there will be a spot on the mountains that merges with the Legate's camp, jump down and go for the tent. IMPORTANT! If you're vilified with the Legion, be prepared for a fight!
There is another method to reach the camp without having to learn the item jump trick, involving an area near Crescent Canyon West where you can just walk right out of the map, However, it takes a rather long time as you have to follow the length of the map template far southeast to where the river actually ends, and come back around on the side of Legate's Camp. Once you reach the camp, it is surrounded by an invisible wall far out enough to be unable to activate the map marker, except for one small location near Legate's War Tent where you can drop down between the rocks and fence, where you can activate the map marker and then be able to fast travel to the camp whenever you like.
You can also reach this place by getting out of the map in the Searchlight Airport. It seems quicker to get to the Legate's Camp by getting off the map in here as it is nearer to the river. Just follow the stream of the river until the river ends. However, don't jump onto the water as you won't be able to get up to the land again. There will be an invisible wall around the camp and there will only be a small spot beside the Legate's War Tent to jump down and discover the place, (see the Searchlight Airport page for more details of how to get off the map).
A quicker route is to exit the playing area at the north east rim of the Devil's Throat. Using your Pip-Boy world map as a guide, head south until just east of The Fort. Then head west until you see the gate between Hoover Dam and the Legate's camp. Walk along the barrier above the gate towards the dam. Eventually you will go through an area change, (similar to fast traveling), to the east side of the gate where you will appear next to 2 Legion guards. Go through the inner gate and you will unlock the location so you can fast travel in and out. If you get too close to the dam, you will go through the same kind of travel, but end up on the dam, east of the check point. You will find one Legion guard and one Legion mongrel. If you head west, you will be able to enter the checkpoint. If you exit the checkpoint through the west door, you will not be able to reenter it, and will be at the NCR controlled portion of the dam.
Another possible method is to exit the area just above the entrance to Bloodborne Cave, then travel east to the river. On your Pip-Boy world map half way down the left side of the map there is a circle with a cross in it. That circle lies next to a small inlet of water on the river that allows you to walk up out of the water and be across from the Legion Ghost camp, and just slightly north east of the Legates Camp. Then walk south along the massive stone wall and until you see a large rock with a rectangle missing from the bottom, (Do not go through this rock, you're feet will become trapped and you cant get out). Walk PAST that rock and climb over the one next to it. Then turn north east and you should see a large sandy slope that lead up to the top of the rocks surrounding the camp. Walk up that slope, then go north east towards the gate. (Do not climb off of the top, there are barriers and you'll get stuck). Just keep walking along the barrier closest to the camp and eventually you will be 'relocated' down to the east side of the gate and you will spawn by 2 Legion soldiers. Your now inside the camp area. Walk down the path to the inner gate and you will have unlocked the location so you can fast travel to and from. (Confirmed on Xbox and PS3)
If you use any of the above methods to get off the map, you can also visit the Legion Ghost Camp which stays between The Fort and the Legate's Camp. You can only access it by getting off the map's limits.
If you reach the Legate's Camp early, Legate Lanius will not appear. He only appears at the final mission of the game.
If you use a glitch to get into the Legate's Camp and kill the legionaries you find, and then return in the final quest of the game, the legionaries will not have respawned, leaving a clear path to the Legate.
PC Pathfinding when Oliver and the rangers blow down the door to the Legate's camp (and when any Securitrons show up after) is poor and causes everyone to get stuck outside the camp.
PC If standing next to the door (left side when facing out of the camp) when the Legate is killed (and the door blown out by Rangers) you can become trapped without the ability to access Pip-Boy or anything else. (First occurrence also had Courier leveling up by killing the Legate). [Needs Confirmation]
It is possible to get completely get out of the map from this location. If you use 2 boxes of 5mm rounds on the right side the Legates tent between the rock wall and wood crate. Once on top of the tent make an ammunition wall or use complimentary vouchers to get over the invisible wall, but don't worry its a very short wall. Then go behind the tent and work your way up the wall, you may have to make another ammunition hill because there is another small invisible wall. But from there on there are no more walls except for the ones around the main area. (Confirmed on Xbox)
Xbox 360 When walking up the road to the camp an option is given to "Open gate to Legate's camp" . It is in the middle of the road and causes the game to freeze when activated.