In the historic drawing of the old castle garden, a dry pond, the "Oshiro Oniwa Ezu", is depicted with a large boat-shaped stone on the northern shore and an island of rocks in the middle. In an excavation survey, the large boat-shaped stone was not found, but it is believed that the island lies under three rocks that can be seen in the pond. The original pond is believed to have been deep, surrounded by sturdily piled rocks, and exceptionally large in scale, much larger than depicted in the drawing.