Ishigaki are one of the most impressive features of any castle. The skill it took to make them from cutting and collecting the stones to actually building the walls in a myriad of shapes, terrains, and locations, is truly remarkable. The fact that there are still so many stone walls remaining after hundreds of years attests to the skill of their builders too. You can even read the history of a castle from its ishigaki. The type of stone tells you where it came from, markings on the stones tell you who it was cut or gathered for, and the method of building the walls can also tell you in what period they were constructed or by whom. Even in the same castle, you may see walls constructed with different methods indicating who built them and when. You can also find unique stories buried in the stone walls at many castles like the old woman who donated her grinding stone or Buddhist statues that were procured from temples to fill in the walls.
Below is a description of the main styles and types of stone walls. There are also a few sub-categories and rare types not discussed here that I may put together for a future page.
The pictures below are from a display at Kanazawa Castle that show how the stone walls are constructed. You can see that the stones are much longer and larger than they appear from the outside. Smaller stones are filled in between the larger stones to stabilize them. Extra stones are backfilled behind them for drainage to help prevent erosion and smaller stones are also filled into the spaces in the front to stabilize the larger stones. Note that there is no mortar used. This allows the stone walls to have some flexibility which is what has helped them to survive for hundreds of years in earthquake prone Japan.
The style of wall indicates the pattern of how the stones are arranged. These two patterns below are used with all of the different types explained in the next section.
The stones used are of various sizes so that there is no particular pattern to the face of the stone wall.
Most of the stones are roughly the same size so that they line up across the face of the wall.
Stone walls can be categorized by how much the stones are processed to fit together in the wall. Walls that use unprocessed stones tend to be older, while walls that make use of more highly shaped stones are newer making use of newer techniques and tools.
These walls use unshaped stones. They are either stones that were used in their natural shape or were split without any further shaping. This type of wall is comparatively weak and high walls cannot be built. It also provides many footholds and handholds making it relatively easy for attackers to climb.
The stones are pounded tightly into place and the corners and rough edges are smoothed some to help them fit together better. The face of the stone is also chipped away to make it more flat. The remaining holes are filled with smaller stones to tightly fill the gaps. This makes a stronger wall than Nozurazumi and these walls can be built higher.
In this type of wall, all the stones are heavily processed and shaped to fit exactly with the surrounding stones. There are almost no holes or empty spaces and no smaller uncut stones filling the spaces between stones. Since water cannot seep out through the spaces they often needed to build in some holes for drainage.
This is basically a special type of Kirikomihagi where all the stones are cut with five or six sides and fitted together.
Tanizumi / Otoshizumi
The stones are fit together at an angle so they have a diagonal pattern instead of horizontal. It takes advantage of the weight of the stones to hold them in place. This construction was actually comparatively easy and often used in the later Edo Period.
This method uses river stones that were naturally shaped by the river to be round. The only castle I know of that makes use of this is Yokosuka Castle in Shizuoka Pref.