This is the most time-consuming achievement in the game. You have to access every single square of both the Normal and Inverted Castle. There is an excellent map HERE that you can use to keep track of your exploration. You'll need to open it in Excel (Read Only) to view it.
If the achievement doesn't unlock by the time you fight Shaft in the Clocktower of the Inverted Castle, then you've missed a room, so use a Library Card to teleport out of the fight with Dracula and return to the Normal Castle. From there, study your in-game maps to figure out what you missed. There are a total of 1890 rooms in the two castles combined, 942 in the Normal Castle and 948 in the Inverted Castle.