You've proven yourself to be a true friend of the Legion. We have a safehouse available to our agents in the Mojave - here's the key. It's an excellent place to rest and store extra equipment. In addition, one of our veterans, Atticus, visits the house every few days. While he's there, you can acquire items useful for stealthy operations against our enemies from him.”— Lucius
Continue doing assignments given by Caesar's Legion members and earn their respect. Reaching the status of 'liked' yields the key to this safehouse from Lucius, leader of the Praetorian guard. At this safe house, there are multiple sets of legion faction armor and some weapons. The safehouse is made of two rooms connected by stairs.
Reaching a "liked" reputation can be accomplished multiple ways; one of the easiest may be to turn in 50 NCR dogtags to Severus or Aurelius of Phoenix. The Courier can also turn in a smaller number and complete a few Legion quests, which may require less time than gathering 50 dog tags. Another alternative, if the Courier is going to side with the NCR, is to pick pocket the dog tags off of live NCR troopers, since this causes no NCR fame loss and only minor Karma loss if it is done while remaining undetected. Also, having the NCR emergency radio makes it possible to call an NCR trooper, pick pocket the dog tag, wait 24 hours, and repeat.