The ruins of a Sengoku-period (Warring States) naval base of the Later Hojo clan, now a national historic site laid out with paths. It appears in episode 6 of the first season, and the city's official manhole map shows a Love Live! Sunshine!! cover here. From the top there is a view of Mt. Fuji across the bay.
Please be a considerate guest: keep your voice down, keep paths clear and leave the place as you found it.
