In the last stage, before you climb the stairs to Dracula, jump down to the left. You will land on an invisible platform. Move all the way to the left of the screen and you will get full health, 99 hearts, a cross, a full upgraded whip, and a III stone.
Hard Mode
Beat the game once.
Three Hidden Routes
There are three hidden branches from the main path in this game.
The first one is in stage 3-1. There will be a point where you must descend a short vertical shaft. Halfway down here is a wall of pillars to the left. These can easily be demolished, where you will find a small side room containing hearts, cash bags, and some Roast.
The second secret, which is an actual hidden Block, is in stage 6-2. At the very beginning you'll need to pass a hallway with Axe Knights and falling chandeliers. The third archway has a chandelier but no guard. Hit the floor a couple of times and it'll crumble, revealing a stairwell into a secret area.
The last one is in Block 9-2. The third section has you jump across a bed of spikes at the beginning. After this you'll see several whirlwind tunnels and some coffins. The last upper platform on the right has a coffin and a tunnel. Let the tunnel suck you in and you'll find a hidden area full of bonuses.