Having completed the perimetric structures, the design turns inward; the next element is the room design.
The castle has eight identical rooms on each floor, there are no additional spaces other than the small openings inside the towers.
The major diagonals of the plant octagon outline eight wings in the castle that are triangular in shape; one of the wings is highlighted in folio 115:01.
A uniform and symmetrical disposition of the spaces suggests a room of identical shape and size in every wing. The room shape has to be trapezoidal, because of the funneling form of the wing that converges toward the center of the plant, folio 115:02.
Therefore, each room in the castle has the shape of an equilateral trapezoid, folio 115:03.
The two slanted sides of the room are formed by the dividing walls that run along the plant octagon major diagonals, folio 115:03.
The major diagonals delimit the length of the room, which is reduced by the thickness of the dividing walls.
With the room space starting at the interior face of the façade wall, the major base of the room trapezoid is already defined in location and size.
With three sides of the room trapezoid defined, what remains is the location of the minor base of the room trapezoid; the height of the trapezoid is the width of the room, folio 115:04.
The same geometric design process serves to draw the room space in each of the eight wings of the castle. The resulting rooms all have the same shape and length, folio 115:04.
The remaining unknown is the trapezoid height, which is the room width.
The room width also defines the indoor face of the courtyard wall.
The room width is dependent on and follows the design of a vault for the trapezoidal room.