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We often use the idiom "copper walls and iron walls" to describe the solidity of buildings, but the Xiyu Ammunition Main Depot is a genuine "wall of copper and iron" indeed! The Xiyu Ammunition Main Depot comprises the earthen cavern cooling ammunition depots and the grotto-like ammunition depots commonly referred to as "copper walls and iron walls." These are rare military structures from the early Japanese colonial period and one of the four major ammunition depots of the Japanese army in the Penghu Islands. As the name implies, the Xiyu Ammunition Main Depot was primarily used to store gunpowder and artillery ammunition. The grotto-type depots feature single and double cavern depots for storing smoky and smokeless gunpowder, respectively. These ammunition depots are constructed with a double-layered vaulted structure. First, a circular arch-shaped cave was carved into the rocky strata within the mountain. The outer depot structure was then built by stacking concrete blocks, while the inner depot space was formed using reinforced concrete. To store smokeless gunpowder, the facilities maintained constant temperature and humidity conditions, and copper sheets were embedded in the walls to completely isolate electromagnetic pulses and moisture. In comparison, the earthen cavern cooling ammunition depot is a ground-level structure. Its outer walls consist of stone walls, while its inner walls featured wooden partitioning and elevated anti-moisture flooring. The roof framework used steel truss construction, with traditional smoky-blackened cultural tiles installed on top, along with lightning rods for protection. These two types of ammunition depots are rare examples of military architecture from the early Japanese colonial period in Taiwan, exhibiting distinctive construction techniques from the Japanese colonial era, and have thus been designated as county-designated historical landmarks for preservation.