Nagano Castle was a mountaintop castle built in present-day Nagano, Kokuraminami-ku and it was characterized by the surrounding moats that ran parallel to each other on the mountain slopes. The parallel trenches were defense facilities where earthen mounds and dry moats were alternately arrayed. In 1565, the local Nagano clan barricaded themselves in the castle to fight against the seiging Otomo forces of Oita.