First, make sure you've gotten both rings from Maria. The first you recieve after defeating the Succubus in the 'Nightmare' stage. The second is past the hall with all the spikes in the Cathedral (you need the SpikeBuster Armor from the Catacombs)
Equip both rings at once in the room with the big clock, and the floor will open. Down in this 'lair' Maria is waiting for you once again. She'll give you a vital piece of information.
Once all this is done, then go to Dracula's Keep and meet Richter. He'll attack you, but you will now be able to see a green orb floating above him. Attack the orb, but _don't_kill_Richter_ (hint: the Sword Companion will serve you well here) After a bit of attacking, the orb will break open, revealing everyone's favorite loathesome 18th/19th century warlock: Shaft! (daaaarn right! right on, bruthaman!)
This also breaks Shaft's spell, freeing Richter from the spirit of Dracula. Alucard will tell Richter and Maria to escape while they can as the clouds swirl and open up, revealing a mirror-image of the Evil Castle! Shaft escapes into the Inverted Castle, and of course it's Alucard's duty to chase after him!
But just when you think you know the layout already, you find some crucial elements of the castle have been rearranged, it's stocked with bigger and badder enemies, and even the music has changed!