Through the ruins of Hara castle, we can witness the early days of the stone wall fortification technique which was in fashion around the time of Oda Nobunaga and Toyotomi Hideyoshi. Castle construction was in a period of transition between medieval and early modern times.
The missionary Alessandro Valignano, in one of his reports to the Society of Jesus, wrote the following concerning Hara castle. Hara is a much more suitable place than where he [Arima Harunobu] currently resides. It is a place where he can robustly defend himself, and so he has already decided to move there. He will build a new castle in this spot.
Through the ruins of Hara castle, we can witness the transition between medieval and early modern times which castle construction was undergoing. On the one hand, Hara’s second and third enclosures are mere hardened mud enclosures which utilise the natural terrain (typical of the medieval style). On the other, the castle's keep has a stone wall, which was one of the features of castles built in the early modern period.
At the time of the Imjin war (1592-98), Arima Harunobu was ordered by Toyotomi Hideyoshi to help with the construction of Japanese footholds for attack such as Hizen Nagoya castle in Kyushu and Wa Castle on the Korean peninsula. Whilst doing this, Harunobu learnt about the stone wall fortification technique through contact with other feudal lords. Hara castle’s stone walls offer us a glimpse at the early days of this technique (unrefined, straight-lined stone walls were in fashion at the time of Toyotomi and Nobunaga). Before long, however, this technique became more refined and aesthetic, and the walls acquired curves.