It sits on the Isthmus plateau, which is disputed land between the Bafal Empire and Rosalia, it's current owner. The Keep is guarded by many elite soldiers of Rosalia, including Albert and Diana the children of Rudolf, and guards Rosalia from invasion by Duke Cornelio of Loban.
Unbeknownst to many, Isthmus Keep houses a dark history; it lies directly above Queitus, the Altar that Saruin was sealed in.
Albert and Diana lead a campaign to exterminte monsters that are in a cave nearby the keep which posed a threat. Rudolf had historians investigate the cave prior to the campaign due to the fact that it bears strange similarities to the the place of sealing mentioned in historical texts. He reports his finding to King Karl. After returning successful, Prince Neidhart arrives to request Diana's hand in marraige. After the Prince leaves and the celebration begins an army of monsters attack the Keep. Lord Rudolf stays to defend the stronghold while sending Diana and Albert to seek aid from Crystal City. Lord Rudolf, Lady Mariah and the Soldiers of the Keep manage to slay ten times their own number but eventually fall in battle and the Isthmus is lost. Saruin's Minions attacked the Keep to ensure the revival.
After the final battle with the wicked God, his death destroys everything around him, toppling the Keep as well as submerging most of the Isthmus plateau into the sea.