Shimabara Castle is a , located between Ariake Bay and Mount Unzen. The outer moats, some 15 meters deep and between 30-50 meters wide, extended 360 meters east-west and 1260 meters north-south, with the enclosed area divided into three baileys. The walls extended for 3900 meters and had 16 yagura of various sizes at key points. The main donjon had five stories, and a height of 33 meters, and was connected to two secondary keeps, each with three stories. In terms of scale, it was far larger than normal for a daimyō with revenues of only 40,000 koku .