It’s absolutely loaded with bizarre King Arthur-related merchandise from throughout the centuries. The more “off” it was historically, the more Merlin finds it funny, and he’s collected just about everything from lunch-boxes to tapestries.
The top floor is entirely devoted to his laboratory. The center of the tower all the way to that top floor is hollow, allowing the “elevator” (so to speak) to carry passengers to any floor. Other rooms ring the walls, all around the outside, connected by a narrow hall.