Harper Robinson studied and drew these floor plans of every level of Hogwarts castle, taking into consideration the smallest details from the books—read Harper's comments below and be amazed—as well as the conventional construction and design of castles over the years.
There are several different floor plans featured in the books, including a map of the first-year dormitories (which include a room for each student), the layout of the Great Hall where the Sorting Ceremony takes place, and a diagram of the lower levels of the library. However, there is only one true "master" plan that covers all six floors of the castle tower: this is the plan drawn by Harry Potter and Ron Weasley during their secret visit to the castle before its fiftieth anniversary party.
In addition to these detailed drawings you can find notes on how certain rooms were used throughout the years, such as the portrait gallery which was once filled with pictures of family members who had died but which now contains only empty frames because Godric Gryffindor removed all his family portraits so he could fly with Peeves the Poltergeist!
Finally, there are two maps featured in the book. The first is a "sketch" map showing where all the important places are located within the castle grounds. This map is useful for visitors who don't want to get lost while they're looking around.
Hogwarts Castle has seven levels and several towers. In addition, the castle features a plethora of known and unknown dungeons beneath the earth. The exact number of floors is unknown, but it's estimated that they contain around 2 million square feet (180,000 m²) of living space.
In terms of rooms, Hogwarts contains over 500 rooms. It is estimated that there are more than two million square feet (180,000 m²) of space here if all the rooms were placed end to end they would stretch for miles.
The castle has been described as having "seven layers" or "a deep well of magic". This refers to the fact that under most parts of the castle lies a dungeon full of ancient magical creatures including dragons. These dungeons are known as "the lower pits" and can only be reached by certain passages which lead down from the main courtyard.
At the bottom of these lower pits are large vaults which contain more magical creatures, this time dead. There are also three secret chambers located within the castle that are only accessible with help from the Death Eater ministry to which Voldemort now belongs. One such chamber is located behind a painting in Professor Snape's classroom. If you look closely at the painting, you will see a small crack on the left side.
So, from the outside, Castle Howard has three major levels: ground, primary, and higher; nevertheless, there is a subterranean cellar area in one wing of the building, as well as an attic floor in the center block. The grounds also contain several other buildings including a coach house, stables, and a gardener's house.
Inside the main building are 72 rooms on three floors. The ground floor contains service areas such as pantries, livery stables, storage, and workrooms. The first floor is where most of the guest rooms are located along with a large drawing room, several smaller sitting rooms, and a dining room. The second floor has more private bedrooms and bathrooms, as well as a gallery, study, and two large bedrooms used by members of the family when they visit the castle.
There are also three basement levels at Castle Howard. The first is an unfinished storage area, while the second and third comprise a single room with a kitchenette. There are no public access points to any of the basement levels.
In conclusion, Castle Howard has three main floors with 72 rooms each. There are also three lower levels with a total space of about 12,000 square feet (1130 square meters).