A huge rock under the remains of the keep foundation on the south side is called Uma-arai iwa (meaning "horse washing rock"). It says in the illustration that the circumference of the rock is 23 "ken" and 2 "shaku", that is, about 45 m (147 ft). It is a living granite. When Naegi Castle was attacked by an enemy and the water supply was cut off, the worriers put a horse on the rock and wash the horse with rice to deceive the enemy, making it look like they have an abundant supply of water. That was where the name of the rock came from.