Japanese castle, unusual pentagonal Tenshu

The Japanese castle towers are usually rectangular constructions, but as you can see in today's photo, there is a notable exception at the Okayama Castle, whose tenshu was built with an unusual, pentagonal shape.

It's not easy to spot this shape, because the tip of the pentagon is located on the opposite side of the entrance and there is no alley running next to that wall, but the pentagon is visible more clearly in GoogleMaps. Besides the aesthetic purpose, the main reason for this shape was to increase the resistance of the basement wall.