It is one of two halves required to activate the Grand Lift of Dectus, with the other half being in Fort Haight in Mistwood. There is a chest here containing the Medallion. Once you reach the back of the room, climb the ladder to reach the top of the tower. Beware as they are significantly tougher than any you've encountered in Limgrave.
To get the Dectus Medallion piece, head inside the main entrance and make your way through the dark room, fighting off the Giant Bats and Chanting Winged Dames. Of note in this fort are the Dectus Medallion (Right) and the Radagon's Soreseal talisman.
