"The Art of Glitching, Part 2 of 11: Underground Gardens Corridor"—These series of gamechives are dedicated to some of the more well-known glitches and tricks that have been found in the game. These are not exhaustive by any means, and some of these methods vary by platform, but it's amazing the lengths that players have gone to increase the room percentages on the map or to find ways to circumvent the game's programmed events.
This particular video is dedicated to the corridor to the Underground Gardens.
Underneath the slab at the very beginning of the game (right after entering via the drawbridge) is a small two-room vertical cavern with a Save Room. This is where an entrance to an area called the "Underground Gardens" was to have been created, but since that area was never finished in the Playstation version, the slab gate never opens. However, a player can get in there using either the "Wolf Dash Trick"* or "Double Heart Refresh"** (easier to pull off) glitch!
Go to the first room after the castle entrance (immediately after the room with the slab gate on the ground). Either Wolf Glitch the leftmost slope or Double Heart Refresh the left screen, then Wing Smash over to the left once under the room's ground. The player will then end up underneath the ground in the beginning of the game, and once traveled far enough, the player will be pulled into a glitched—but functional—Save Room. From there, jump up through the tunnel and out through the slab gate, which will now be open!
*This method utilizes the Wolf form, where a player find a stairway, and while in wolf form, dashes toward it. Right before the stairway is reached, tap the jump button and transform back into Alucard. If timed correctly, the player will be stuck under the floor.
**This method requires equipping a "Heart Refresh" item in both hands, and continually using one after the other in a successive motion, until the player is slowly inched off the screen and into the next room.