Stier Castle appears inAce Combat 5: The Unsung War and Ace Combat Zero: The Belkan War. Positioned on the southern edge of the borders on between the Principality of Belka and North Osea, (formerly South Belka; 27°01'18"N 230°12'02"E) this castle overlooks the seven ground zero nuclear explosions set off by Belka on their own cities during Belkan War. The local area is still believed to be highly radioactive. The geography of this area is a large, flat valley floor surrounded by the Waldreich Mountains Range. The is evidence of glaciation in this area, the very mound that Stier Castle stands is an old and short drumlin, a hill that has been stretched out by glaciation. The large lake in front of the castle was probably made by the same glacier that reshaped this region.
Stier Castle, which is near the Waldreich Mountains was once the largest castle in Belka during their Dark Ages, well before they modernized. Many of the honored Belkan Knights were said to have lived in or passed through Stier Castle during those times. It is also believed that the popular Belkan Story "A Blue Dove for the Princess" is set in Stier Castle, in the surrounding area and in an area of the Waldreich Mountains. In modern day Belka, Stier had become a national Belkan symbol and a tourist attraction. There are quite a few towns on a road leading north from the castle, apparently well mantained, but presumably abandoned since the radiation from the Belkan nuclear bombs have rendered the region virtually uninhabitable.
On June 6th, Stier had witnessed the explosions of one of seven Nuclear Weapons, across the Waldreich Mountains. It survived the blast, miraculously. Stier also had witnessed the destruction of the XB-0 Hresvelgr when "A World With No Boundaries" used it to bomb Valais Air Force Base. As it flew towards North Belka it was intercepted by the Galm Team and destroyed. It crashed miles away from Stier Castle and the Nuclear Blast Site.
During the Circum-Pacific War in 2010, Osean President Harling was imprisoned in the Stier Castle. On December 9th, 2010 at 0500 hours, the Razgriz and the helicopter squadron Sea Goblin launched a rescue operation for President Harling, from the Air. This was known as Operation: Ancient Walls. As the Razgriz held off opposing Belkan forces, an SpecOps team dropped & infiltrated Stier Castle. A close firefight broke out between the Osean and Belkan Forces, and the soldiers moved through the castle. At the same time, a combined force of Tanks, mobile AA cannons and mobile SAM Launchers began surrounding the castle. The Razgriz held off the Belkan forces, but were subsequently attacked by Belkan fighters; this was unexpected, since Belka was not allowed to possess aircraft after the Belkan War. The ground forces managed to secure the President, but were unable to fight their way back out to the Sea Goblin chopper. The Razgriz destroyed one of the walls of the castle to allow the infiltration team to escape. The operation was successful and despite damage, Stier Castle was still standing.
In addition to being the center of the mission 20 ("Ancient Walls / Operation Keynote") map in Ace Combat 5, Stier Castle can be spotted below the starting point in mission 12 ("The Stage of Apocalypse / Operation Ravage") of Ace Combat Zero. Later in that mission, the lower half of the map is cut off by the nuclear explosion, and it is the crater left by this explosion that President Harling reminisces about in Ace Combat 5. Also in Zero, in mission 15 ("The Talon of Ruin / Operation Valkyrie"), if you fly to the lake in the midpoint of the map, Stier Castle can be seen on the edge of the lake.