The Rock Center or just the Rock, more precisely the Rock Center of Little Gellért Hill, originally known as the Citadel (Fellegvár), is a mostly subterranean military complex in the 11th district (Újbuda) of Budapest, Hungary. It is under Little Gellért Hill, the Western extremity of the well-known Gellért Hill. It is also known by the codename Deep 23. The complex hosted the National Counter-Air Center and thus it is also known by that name. The Rock Center should not be confused with the nearby Citadella, which was never called Fellegvár, but which also has a military bunker, the Gellért Hill bunker under it.