After the Shimabara-Amakusa Rebellion, Yamasaki Kainokami Ieharu became the lord of Tomioka Castle. In order to strengthen the castle’s defenses, he constructed an embankment at the place where a cove was to dam the sea tide, moved the houses which were located on the inside of the embankment, and created an inner moat which he filled with river water. This embankment is 170.1 meters long, the path along its top is 4.7 meters wide, and its stone wall is 8.1 meters high. Under the north side, there lies Fukuroike Pond. A water channel was built at the end of this embankment to keep the moat’s water level constant. The hallway bridge built over this water channel was designed so it could be broken in times of war.