Go to the Sky walkway area of the castle and make your way to the save room, which is near the enterance to the Clock Tower. Equip your crushing stone and power up your charge attack. When powered up, jump add blast the block of bricks on the opposite end of the room. You have now opened a hidden room. In castle B, the room contains 2 medicine jars. In castle A, it'll contain the Heart of Vlad.