逆井城Sakasai Castle is classified as a flatland castle, and is located in Ibaraki Prefecture. During the pre-modern age, it found itself within the borders of Shimōsa Province. It is associated with the Sakasai, late Hōjō clans. Dates in use: 1456 - 1590.
I haven't actually visited this castle myself, but from what I've heard it is not easy to get to. Once you arrive however, your efforts will be repaid with a finely reproduced Sengoku period castle. Prior to being abandoned in 1590, this castle marked the eastern border of the Hōjō territory.