Keystone is the codename of President Benjamin Hathaway's hidden location.
Keystone is a secluded air base in the mountains located somewhere in New York or Pennsylvania.
Tom Mason, Cole Bressler, Cochise, and John Pope traveled there to meet the President. They were attacked by Espheni soon after their arrival. The location was revealed by Lourdes through a tracking device she placed in the med kit Anne gave Tom.(At All Costs) It is likely the base was destroyed by Airships after the Espheni learned of its location.