The remains of a north culvert (北暗渠), or drain, located outside the garden, depicted in "Oshiro Oniwa Ezu", were found in the course of an excavation survey and faithfully restored. The drain is thought to be the remains of a stone culvert for channeling rainwater, as mentioned in the "Kinjō Onkoroku" (金城温古録) document. Even today, rainwater is channeled to the moat through this drain. The stone materials of the culvert include granite for the lid and hard sandstone for the sides. According to the drawing, there was also a flower bed nearby.