The Chinese idiom “copper walls, iron bastion” is often metaphorically used to describe a sturdy building. However, the Xiyu Ammunition Depot is a literal embodiment of that phrase! The site includes a bunker-style cold ammunition depot and a cavern-style ammunition depot with copper-lined walls. A rare military installation established in the early Japanese era, the depot was one of four Japanese ammunition storehouses in Penghu.

The Xiyu Ammunition Depot was primarily used to store ammunition for artillery. The cave-like depot is a double-walled dome structure with two dry ammunition storage spaces, single-cavity and double-cavity, one for regular gunpowder and the other for smokeless gunpowder. Builders first dug out an arched cavity in the mountainside bedrock, then constructed the outer storage area with concrete blocks and the inner storage area with reinforced concrete. Storage of smokeless gunpowder required strict control of temperature and humidity levels, and walls had to be lined with copper sheets to keep out electromagnetic waves and moisture.

The cold ammunition depot is an above-ground bunker. The outer walls are built of stacked stones, and the inner walls and floor are wooden; the floor was raised for moisture-proofing. Japanese-style blackened tiles cover the steel roof trusses. The roof is equipped with lightning rods. The depots are rare military architectures dating back to the early Japanese era, displaying the construction technology of the time. The site was designated as an official Penghu County monument for preservation and restoration.