There are three items hidden in the castle that serve no purpose except as inside-jokes and trophies.

The Hidden Bell - Located in the Dark Chapel in the bell room. Using the Hippogriff soul super jump into the first bell, doing so a special bell will fall out.

The Hidden Crown - Located in the huge vertical portion of the Demon Guest House the part with the chandeliers. Head up to the room in the southeast corner that contains a single Waiter Skeleton and a chair. Sitting in the chair for a few seconds and a crown will emerge from the ground, just like the original Castlevania.

The Hidden Konami Man - The old school mascot for Konami is located in the Cursed Clock Tower. Visit the room with a single Slime and Tanjelly and then crouch in the spike-free area for one second. Konami Man will fly across the screen in all his 8-bit pixelated glory.