For those who have the Bell item but it doesn't work as intended (like what happened with me), there are only six items that really need your attention, all of them in the castle itself (and not in the dungeons). One of them requires killing every enemy in one specific room, but you can only do so later in the game. The other five items are in hidden walls. Here are spoiler-free tips for their locations:

1: In a room with moving blocks;

2: In a room with a big hole and a staircase;

3: In a symmetrical room;

4: In a room below the Devil's dungeon;

5: In a room with magma.