Kamakura is riddled with small caves carved into the soft rock. These caves are tombs, built in medieval times. According to Wikipedia, Kamakura has anywhere from 1500 to 5000 yagura cut into its hills. It is assumed that many have yet to be found.
The yagura pictured is Kamakura’s largest, located at Meigetsu-in in Kita-Kamakura.