The Playground was a secret base of unknown generation, operated by Nick Fury. Its original purpose was classified. It was located on the borders of Syria, Iraq, and Saudi Arabia, suggesting there were valid reasons for its secrecy.
Marvel Cinematic Universe (Earth-199999)
As with Earth-616, Playground is one of Fury's Secret Bases, the name coined by Billy Koenig. The base formerly belonged to the SSR, in which Agent Peggy Carter hid a great many classified documents in concealed vaults.
The Playground was eventually captured by a splinter S.H.I.E.L.D. faction led by Robert Gonzales, though Coulson escaped through a hidden elevator. Coulson managed to reclaim control of the base after he and Gonzales' S.H.I.E.L.D. factions came to a merger agreement.