Question: The Festning Tunnel runs under what type of structure?
Context: The Festning Tunnel (Norwegian: "Festningstunnelen" ) is a motorway tunnel on European Route E18 in the city center of Oslo, Norway.  The tunnel has two tubes, with three lanes in each.  It runs from Bjørvika, under Akershus Fortress, The City Hall Square and Vika to Filipstad.  The tunnel is 1800 m and -45 m elevation at the deepest. The Skaugum Tunnel (Norwegian: "Skaugumtunnelen" ) is a 3790 m long railway tunnel in Asker, Norway, on the Asker Line.  The tunnel runs between Asker Station and Solstad and was built as part of the first stage of the Asker Line, between Asker and Sandvika.  Construction started in February 2002 and the tunnel opened on 27 August 2005.  The tunnel was built by Mika for the Norwegian National Rail Administration using the drilling and blasting method with one crosscut.  During construction there was one blasting accident.  Since the tunnel opened, there have been problems with leaks damaging the infrastructure.  The tunnel has double track, is electrified and allows for a maximum speed of 160 km/h .  The cost to build the tunnel, excluding the infrastructure, was 450 million Norwegian krone (NOK).  The tunnel has accelerated intercity and regional traffic west of Oslo and freed up capacity for the Oslo Commuter Rail on the old Drammen Line. The Bjørvika Tunnel (Norwegian: "Bjørvikatunnelen" ) is a motorway immersed tunnel on European Route E18 in the city center of Oslo, Norway.  The tunnel has two bores, with three lanes in each.  In the west, it connects to the Festning Tunnel at Akershus Fortress and runs under the Bjørvika arm of the Oslofjord before ending in an intersection on the east shore, where it splits into Mosseveien (E18) and the Ekeberg Tunnel (National Road 190).  The tunnel is 1100 m long, 675 meters of which run below sea level, and opened in September 2010.  It was built by the Norwegian Public Roads Administration The Bjørvika Tunnel is part of the Opera Tunnel complex which is the name of the interconnected system of tunnels between Ryen and Filipstad The Bjørvika Tunnel is the first immersed tunnel in Norway. 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.  It has also been used as a palace and as a prison. The Mastrafjord Tunnel (Norwegian: "Mastrafjordtunnelen" ) is a subsea road tunnel in Rennesøy municipality in Rogaland county, Norway.  The 4424 m long tunnel runs under the Mastrafjorden, which flows between the islands of Mosterøy and Rennesøy.  The tunnel opened in 1992 as part of the European route E39 highway and the Rennesøy Fixed Link (which also includes the Byfjord Tunnel).  The southern end of the tunnel is located near the village of Askje on Mosterøy and the northern end of the tunnel is in the village of Vikevåg on Rennesøy.  With a maximum grade of 8%, the tunnel reaches its deepest point at 133 m below sea level. The Byfjord Tunnel (Norwegian: "Byfjordtunnelen" ) is a subsea road tunnel in Rogaland county, Norway.  The 5875 m long tunnel runs between Grødem on the mainland in Randaberg and the island of Sokn in Rennesøy municipality, running underneath the Byfjorden.  The tunnel was built as part of the Rennesøy Fixed Link project which opened in 1992 with the goal of connecting all the islands of Rennesøy to the mainland.  The tunnel was a toll road from 1992 until 2006.  The tunnel is part of European route E39 highway.  With a maximum 8% grade, the tunnel reaches a depth of 223 m below sea level at its lowest point.  The tunnel held the distinction of being Europe's longest and the world's deepest tunnel upon its completion, it was surpassed by a deeper tunnel in 1994 and a longer tunnel in 1999.  The tunnel had an average daily traffic of 9,675 vehicles in 2015. The Hægebostad Tunnel (Norwegian: "Hægebostadtunnelen" ) is the fifth longest railway tunnel in Norway.  It is located in the municipalities of Audnedal and Hægebostad in Vest-Agder county.  The 8.474 km long tunnel runs between Audnedal Station and Snartemo Station on the Sørlandet Line.  It was opened in 1943 when the Sørlandet Line was extended west all the way to Moi Station.  The tunnel runs through the mountains virtually the entire distance between the stations of Audnedal and Snartemo.  The tunnel has a 0.2% horizontal gradient and goes straight except for a curve at entrance on the Audnedal side. The Ellingsøy Tunnel (Norwegian: "Ellingsøytunnelen" ) is a subsea road tunnel in Ålesund Municipality in Møre og Romsdal county, Norway.  The tunnel runs between the center of the city of Ålesund on the island of Nørvøya and the village of Hoffland on the island of Ellingsøya.  The 3481 m long tunnel runs under the Ellingsøyfjorden and it is part of National Road 658 and it was constructed as part of the Vigra Fixed Link project which connected several outlying islands to the city of Ålesund and the mainland of Norway.  The tunnel opened in 1987 and it was a toll road until 25 October 2009.  The 3-lane tunnel has a vertical clearance of 4.6 m and it reaches a depth of 140 m below sea level.  The steepest parts of the tunnel do not exceed an 8.5% grade. The Boßler Tunnel is a railway tunnel currently under construction as part of the new Wendlingen–Ulm high-speed railway.  The tunnel is approximately 8806 m long, built as two parallel single-track tunnels.  The tunnel runs between Aichelberg and Mühlhausen im Täle under the "Roter Wasen" and the Boßler mountain.  It is one of a series of tunnels underneath the Swabian Jura range.  When completed, it will be the longest tunnel on the line and the fifth longest tunnel in Germany (after the Landrücken Tunnel, the Münden Tunnel, the Filder Tunnel and the Katzenberg Tunnel). The Bømlafjord Tunnel (Norwegian: "Bømlafjordtunnelen" ) is a subsea road tunnel under Bømlafjorden which connects the island of Føyno in Stord to the mainland at Dalshovda in Sveio, Norway.  The tunnel is 7.82 km long and reaches 260.4 m below mean sea level.  It carries three lanes of European Road E39 and is part of the Triangle Link, a fixed link which connects Sunnhordland to Haugaland.  Plans for the tunnel arose in the 1980s; construction started in 1997 and the tunnel opened on 27 December 2000.  The tunnel was built using the drilling and blasting method, with two teams building from each end.  The tunnel runs through an area composed mostly of gneiss, phyllite and greenstone.  The tunnel is the longest subsea tunnel in Norway and was the second-deepest in the world until 2008.  It is still (2013) the deepest point on the E-road network.  The tunnel was a toll road from the opening until 30.  April 2013.  In 2012 the tunnel had an average 4,084 vehicles per day.
Answer: a medieval castle