In Rondo of Blood:
-Climb the staircase to the throne room. Stand to the left of the moon, on the first two blocks of the platform, and jump straight up and hold UP on the controller.
Richter will land on 'invisible stairs' which you can climb upwards to some goodies (mostly money). If you keep on dying at Dracula the goodies may get you one more life.
In Super Castlevania IV:
-Instead of climbing the stairs to the throne room, take a leap of faith to your doom right off of the platform that has the bottom of the stairs. Instead of plummeting to your death, you'll land on an invisible platform. Keep on walking left and you'll drop to another platform by the wall of the Throne Room, at which point you'll get a bounty of 99 hearts, some chicken to fill your health, a Boomerang, and Double Shot, and a Triple Shot. Getting back up to the stairs is tricky though, as you don't know where the platform ends. You press UP a tile or two from where you got the treasure, and Simon will walk up 'invisible stairs' to the mid-air invisible platform. Jumping back is tricky as you need to have learned by now how far Simon can jump and jump from around the maximum length, back to the staircase platform.
Castlevania 64/Legacy of Darkness:
-Instead of entering the door with the Dragon Emblem at the top of the stairs to the Throne room, jump to where the torches are. You won't be hurt by the torch's fire. Instead, you'll land on the platform the torches make, and will be able to walk around the outside of the throne room. On the opposite side, there's an invisible platform you can walk across to a faraway candle holding goodies (I think it's a Medical Kit, the strongest healing item). I forget if there was more.
In Portrait of Ruin:
-The room just before Dracula's Throne Room at the keep holds a secret on the top. However, it's not hidden at all.
In Harmony of Dissonance:
-The keep stairs have a gate that you can destroy with the Crush Boots. It's at the top of the stairs.