When you reach the Castle Keep, you'll see the Leap Stone above you. Circle around and retrieve it, then use the warp room to leave, because there's nothing else you can do here for now. Your next destination is the Colosseum, and the trek to get there begins in the center of the Marble Gallery.
Once you have the Bat Form, you can challenge Richter Belmont at any time. However, if you first get the Holy glasses and equip them during the fight, you'll notice a floating green orb. Destroy this rather than Richter and you will reveal the path to the second half of the game—the inverted castle!