Matsumoto Castle's hidden floor

One of the National Treasures of Japan, the keep (tenshu) of the Matsumoto Castle is one of the only twelve Japanese castles in the original form and, although it underwent two renovations in the beginning and in the middle of the 20th century, its interior still looks like in the 16th century.

Looking at the castle from outside, it clearly has 5 floors. However, the castle's architecture manages to perfectly hide the fact that on the inside there are... 6 floors! Can you tell which is the hidden one?
The hidden floor (or the "dark floor", kurayamijuu) of the Matsumoto Castle is the third floor, with no windows and completely hidden from the outside. Mainly used for storage, the hidden floor was also a strategic asset against the castle's attackers...