Tsuru-no-maru Dobei (Tsuru-no-maru Earthen Wall) consists of the Namako Kabe (literally "the Sea Cucumber Wall"). This kind of wall is made by attaching flat tiles to the earthen wall with plaster. It is very thick and filled with pebbles, so gun bullets cannot penetrate it. In fact, it works very well for defense. Even if a bullet makes a hole in the wall, pebbles will fall down from the upper part and fill it up right away.