Because the main role of the castle was military defensive (the same Matsumoto Castle has an entire floor hidden), such an addition, designed only for relaxation, was very unusual and there were only a few Japanese castles with a turret like this.
Used especially during Tsukimi (autumn moon-viewing festival), it is said that from this place you can see the moon for three times at the same time: once in the sky, once reflected on the water and once... reflected in the sake cup. (^_^)
EXIF info:
Nikon D300
Lens: 18-70mm F/3.5-4.5G
Focal Length: 40mm
Aperture: F/5.6
Shutter Speed: 1/320s
ISO Sensitivity: ISO 320