You have to activate RSM (Richter Special Mode) in an Alucard file that is 201.2% or higher. Then do a soft reset (select+start) and load a Richter file. Then go to the top of the stairs in the Castle keep and jump straight up and when you land on the flat-top of the stairs, you will fall through and be able to get extra rooms.
http://www.youtube.com/watch?v=xf_z4MaQ26AThe video is pretty easy to see whats going on. kill shades #1,2. bypass #3,4. run.. run.. slam wall. shield dash back jump on the boat. go past the 4th shade. the game should slow down between these 2. now get hit a bunch to make some slowdown. now jump above the 4th shade and turn to bat. you will disapear. This activates RSM!
You can also get outside the Inverted Outer Wall. To get back inside, go to the top right area and jump up and land in the area where you can only see Richter's legs.
Also, if you manage to activate the Infinity Glitch on an Alucard file, and load a Richter file; the gate in the right-hand section of the caverns will be open!! This will give a you a few extra rooms too!
To activate the infinity glitch with Alucard search youtube for "Infinity Glitch (console version)"