From the Slogra & Gaibon boss room, head right into the Marble Gallery. This area is characterized by its grand architecture and numerous pathways. Your immediate goal is to find the Leap Stone, which grants Alucard the double jump ability. This is located in a room high up in the central part of the gallery. Use your new Bat form to fly up to previously unreachable ledges.
While exploring, you'll encounter enemies like Plate Masters and Stone Roses. find the Librarian's room (located near the center of the gallery, accessible by breaking a wall). The Librarian sells items, relics, and hints. Purchase the Castle Map 1 immediately. It's invaluable. Also, look for a Save Room and a Teleporter in this area. The teleporter allows for fast travel between activated teleporter points.