Past the tunnel gate the story goes through the tunnel, the Outlands and the metro to the Central Loop, and you can't walk back: after it, Old Villedor is reached only by fast travel, which opens at the start of the next quest. Nothing here is lost, but finish the first map now: both Houndfield windmills, C-A-22, the Stage IV quarantine, the hospital airdrop, the Bing Bang Boom! skills and trophy, and the Survivor quests above. Left for a later trip: Metro: Hayward Square and Metro: The Holy Trinity (they open only after this quest), the Houndfield Electrical Station and the district quests it opens (stations open from A Place to Call Home on), and anything that needs the Paraglider or the Grappling Hook.