This structure is a Sengoku period castle and its special characteristics are its koguchi (narrow gate) and kirigishi (an artificially constructed diagonally-sloping cliff). This indicates that the lord of the castle, Kukuri, held great authority at that time.
This was the castle of Kukuri, who belonged to a family of Toki shugo daimyo (military governors), and was also a hokoshu (a title given to those involved in military affairs) of the Muromachi Bakufu. In 1582, Kukuri was murdered by Mori Nagayoshi. Following this, Mori Nagayoshi became the lord of the castle. During the second half of the Sengoku period, yokoya, masugata koguchi, and yokobori (moats around the castle) were constructed and it became more complex.