Walls 土塀

Dobei are the white walls you commonly see at castles. They are the simplest and most inexpensive defenses available. Dobei originally lined the top of most moats, stone walls and encircled most of the baileys linking together gates and yagura. Many castles had at least one kilometer of walls and Edo Castle had more than 10km. Despite the fact that there were so many such walls during the Edo Period, if you added up all the extant walls today you would only find a little over a kilometer. The majority of extant walls are at Himeji Castle and the longest single extant section is the Nagabei at Kumamoto Castle. These walls evolved from simple structures of wooden planks nailed to a fence. The walls were strengthened and thickened to prevent arrows from piercing them, to prevent fire and later to prevent bullets from easily passing through.
Himeji Castle Kanazawa Castle Kofu Castle Sakasai Castle Takashima Castle

Structure

Dobei walls are built by erecting pillars approximately 1.5 meters apart. In between the pillars is a lattice of bamboo or wood strips. Mud and clay were then layered over this lattice up to about 20cm thick. The clay was often mixed with some strong Japanese grass (wara) for added strength and to prevent cracks. Earlier forms of these walls were not covered in plaster which gave them a sandy yellow color. In the picture above from Sakasai Castle you can see a wall with no plaster that shows this yellow color and you can also make out some grasses embedded in the clay. Edo Period dobei were usually covered in hard white plaster which increased their strength and helped prevent weathering. Atop the wall they had tile roofs and often had loopholes for firing arrows or guns. Walls also frequently had support posts behind them to increase their strength especially for walls along the top of stone walls or other places where the foundation was not as solid. Some walls also contained strategically placed rock chutes to drop rocks on attackers. Click the pictures below to enlarge these displays of wall construction.
Display at Utsunomiya Castle Display at Kanazawa Castle Display at Odawara Castle
Display at Utsunomiya Castle

Variations

There are some extant variations of these walls that can be divided into neribei and tuijibei. Neither of these have the kind of wooden pillars or interior framework of the usual walls. Neribei are constructed from dried clay bricks or old tiles that are mortared together with clay and covered with a layer of hard plaster. Neribei were employed at Himeji and Bitchu Matsuyama castles to quickly build some walls.

Tuijibei are made from pounding a mixture sand and clay in 3-5 cm layers. They are about 1 meter thick and up to 3 meters tall. They have a distinctive wooden framework on the outside and are topped with a tile roof. These are very strong walls, but their thickness makes it impossible to build in loopholes and they are very time and labor intensive to build. For these reasons they were not commonly used at castles. There is a small section by the Mizu no Ichi gate at Himeji Castle and the Ninomaru of Nijo Castle is surrounded by impressive Tsuijibei.

Himeji Castle Nijo Castle

Loopholes sama (狭間)

Loopholes were holes built into the walls for firing arrows or guns. These are an important defensive structure and we see records of them dating back to the Heian Period. Loopholes designed for arrows were generally rectangles on end and those for firearms were circles, triangles, or squares. Some loopholes were hidden by a door or plug that matched the surface on the outside to prevent detection by attackers. These are called kakushizama.
3 loophole types at Himeji Castle Hidden loophole at Hikone Castle