Dracula's Castle is riddled with hidden rooms, often containing valuable items, equipment, or even entire new areas. Many of these are concealed behind breakable walls, floors, or ceilings. The most reliable way to find them is to attack every suspicious-looking wall. If a wall makes a different sound when hit, it's likely breakable. The Echo of Bat relic (acquired after defeating Succubus) is invaluable for this, as it emits a sonar pulse that reveals hidden passages on your map.
Common locations for hidden rooms include: behind the Librarian's chair, various spots in the Outer Wall, the Clock Tower (by hitting gears), and numerous places in the Underground Caverns and Abandoned Mine. In the Inverted Castle, these hidden rooms are even more prevalent and often contain the best gear. Aim for 200.6% map completion to ensure you've found them all.