If you want a real challenge in Modern Warfare 3 Zombies, the High Threat Zone and Legacy Fortress are the places to be. The latter is a high-security Terminus Outcomes garrison in Urzikstan, and here's how to locate it.
Where to find the Legacy Fortress in MW3 Zombies (Map Location)
The Legacy's Castle can be found in the northwest part of the map that I've marked on the map above. It's located on top of a hill in the Medium Threat Zone between the Popov Power Plant and Levin Resort POIs. I suggest you upgrade your weapons and carry enough armor before going there as you'll face heavy resistance from guards, sentry guns, and even an attack chopper.
How to get the Legacy Fortress Keycard in MW3 Zombies
There's a guaranteed way of obtaining the Legacy Fortress Keycard in MWZ. First, you'll need a Mercenary Stronghold Keycard (first image). This can be obtained from loot caches inside a Mercenary Camp (second image). Alternatively, you can purchase one from a Buy Station for 2000 Essence. Once you have the access card, go to a nearby Mercenary Stronghold (third image).
Clear out the building of any PMC mercs and drill the safe to get it unlocked. While it's drilling, a reinforcement helicopter will bring in more guards that you'll have to take care of. I highly recommend dealing with them afar as the AI with blades can down you instantly. Once the safe is open, you'll find a Legacy Fortress Keycard, 3-Plate Armor Vest, and perhaps something else useful inside.
Stay tuned to Pro Game Guides for more Call of Duty: Modern Warfare 3 content. In the meantime, head over to our article on How to defeat the Mega Abomination Boss in MW3 Zombies.