Main tower base

The stone walls of the main tower base are said to have been erected at the beginning of Fukuoka Castle's construction. They are made with dry rubble work, using small natural stones, and have shallow inclines. They have the appearance of the oldest structures within the castle.

Honmaru

The honmaru [main bailey] of Fukuoka Castle is well-fortified by its surrounding high stone walls. The south and east sides are especially protected by dry rubble walls that are more than 10 meters high, while the north and west sides have walls of course rubble.

Ninomaru

On the north and west sides of ninomaru [second bailey] are high stone walls, while the south side is guarded not by a stone wall but a steep earth mound. On the north side is a high stone wall about 200 meters wide, constructed from split stones.  

Sannomaru

Stone walls can only be found around gates and the bases of the turrets in the sannomaru [third bailey] since the sides facing the moat surrounding the castle are mostly earth mounds. Around the Kamino-Hashi and Shimono-Hashi Gomon gates are exquisite stone walls that use huge split rocks, while the wall at the base of Shiomi-Yagura are built with dry rubble work, using small natural stones. 
 

①Higashi-Gomon (Kagami-Ishi)

Huge rocks are often placed in stone walls that draw attention, such as those near the entrance gates. These rocks are called kagami-ishi and are said to have been set to highlight castle lords' power. At Fukuoka Castle, three grand stones are placed in the wall next to Higashi-Gomon [the East Gate], the official entrance from the sannomaru [third bailey] to the ninomaru [second bailey]. 

➁Ōgi-Zaka (Ya-Ana)

You can find the Ōgi-Zaka-Gomon, a gate that interjects the ninomaru [second bailey] from the east and west. The walls on both sides of this gate use larger stones than others. Another unique feature of these walls is the rocks with traces of ya-ana, holes carved to break the stones.  

➂Main Tower Base (Nozurazumi)

The stone walls of the main tower base are the tallest within the castle, reaching up to 13.4 meters at their peak. The walls use natural stones in the nozurazumi-style, which inclines at a shallow angle. The walls appearance clearly indicates their age. 
It is said that they are the earliest-built structures within Fukuoka Castle. 

➃Beneath the Bugu-Yagura (serving southern defense, along with the earth mound)

The stone wall beneath the Bugu-Yagura [Armory Turret] is 12 meters high and is 75 meters long. Its grandeur is overwhelming and surely would have repelled any enemy. On top of this wall was a grand two-story turret with three-story corner turrets to safeguard the honmaru [main bailey].

⑤Beneath the Tamon-Yagura

On the west side of Tamon-Yagura [Multi-Room Turret] are two layers of stone walls. These effectively make the turret appear taller when viewed from a lower vantage point, creating an intimidating effect on attackers. The lower wall contains many stones with engravings, and more than 20 types of engravings have been found here.  

History of the stone walls

Stone walls were first built at Japanese castles at the end of the 6th century CE. Techniques to construct stone walls came from China and the Korean Peninsula and were applied at forts such as Ōno and Ito Castles. 
During the late 13th century CE, stone fortifications were built in Fukuoka to protect the area from Mongolian attacks. It is said that these stones were later reused to build the walls of Fukuoka Castle. Near the end of the 16th century CE, when firearms became the primary weapon on the battlefield, turrets were structured around the castle for protection, and high stone walls were constructed to place them on top. 

How to build stone walls

The interior of a stone wall must have a specific structure to make it tall and resilient. While we can see large rocks on the surface, the stones become narrower inside. Gravel is laid behind these rocks to allow rainwater to drain. 
Once the large rocks have been stacked up, small stones are placed between them to fix the wall's surface in place. These stones refine the look of the wall; at the same time, they make the wall challenging to climb. 

Breaking the stones 

On the surface of a stone wall, you may see the remains of ya-ana or arrow holes that were made to break the stones. Ya refer to iron wedges driven into the stone to break it. A stone is broken by carving a line of holes in its surface and then driving the wedges into the stone to create cracks.
Some stones are so well-broken that arrow holes remain like gears, while others are unfortunate in that they are left unbroken with only the remains of the wedge holes. 

Types of stone walls at Fukuoka Castle

Stone walls of a castle are made by stacking the stones without using any adhesives such as concrete; a technique known as karazumi. The walls are classified according to the shapes of the stones. At Fukuoka Castle, you can find the following two types. 

Nozurazumi building

Stone walls made by dry rubble work were known as nozurazumi. Precision techniques are required to build these walls, as the stones come in different shapes and sizes.

Uchikomihagi building

Large, course rocks are broken and shaped before being stacked to form walls. The surface of the walls is even, creating a structured look. For this reason, at Fukuoka Castle, they are used at sites that stand out, such as near the gates.