The Great Wall of Suzanoh is seen in Chapter 23 of Conquest. It is a very large wall that protects the citizens of the Hoshidan capital and Castle Shirasagi, and plenty of mounted ballistae are placed on the walls.
During the Conquest path, the Nohrian army attacks the Great Wall of Suzanoh. After Corrin's army takes out the Hoshidan core legion, the Nohrian army captures the fortifications and proceeds towards the Hoshidan capital.
The name "Suzanoh" is based off the name of the Shinto deitiy, Susanoo-no-Mikoto, the Shinto god of the sea and storms.