If you don't go right into the room ahead, the giant spider can be killed with long-range attacks as it's too big to come down the small tunnel. Just before you hack through the web to reveal the giant frostbite spider, there is an adept level chest to your right hidden behind more spider web.

Eventually you must fight a giant frostbite spider and pull an additional lever lowering a bridge allowing access to the courtyard. Once on the bridge take a left as instructed by Serana leading you into an open area inhabited by death hounds, afterwards move up the stairs in the southern section of the room taking you into a tunnel. After dealing with the feral vampire, you will arrive at a stone archway blocked off in the western corner of the room, ignore this for the time being and proceed instead up a set of stairs on the opposite corner of the room, this will lead you to a lever needed to lower the bridge blocking passage through the stone archway. Once you enter through the secret entrance to the castle, you will have to fight your way through death hounds, skeletons, and a feral vampire. When you reach the castle's old docks, you will be attacked by skeletons (more powerful than average skeletons). Serana will mention you're getting close to the entrance.

When you reach the castle, head left, along the coastline. Serana won't accompany you unless you send your follower home, and you'll need her later on. Simply telling them to wait is not enough. Note that if you have a follower at this point, you must dismiss them. You will not be able to enter through the Keep, but there is a secret entrance used by the previous owner to obtain shipments. After going through her dialogue, Serana believes Valerica may have left a clue at Castle Volkihar, in the courtyard. She thinks her mother, Valerica, may have information on it, and possibly may even possess it. Serana will want to speak to you after the Moth Priest reads the Elder Scroll, saying she may know where one of the scrolls are.

Chasing echoes skyrim