The Festning Tunnel runs under what type of structure?
Festning Tunnel:  It runs from Bjørvika, under Akershus Fortress, The City Hall Square and Vika to Filipstad. Akershus Fortress: Akershus Fortress (Norwegian: "Akershus Festning" ) or Akershus Castle (Norwegian: "Akershus slott" ) is a medieval castle that was built to protect Oslo, the capital of Norway.
a medieval castle