During Japan's Sengoku “Warring States” era (1467-1615), castles were constructed, bolstered and fortified all across the Japanese archipelago, resulting in approximately 5,000 individual keeps.

Japan reached its peak number of fortifications during the Sengoku Period (Warring States Period), which lasted from roughly 1467 to 1573. During this era of constant civil war, regional warlords (daimyō) built thousands of defensive structures, ranging from small mountain forts (yamajiro) to massive stone-walled complexes, to protect their territories and signal their power. Historical estimates suggest there were over 5,000 such fortifications across the archipelago at this time. However, this number was drastically reduced following the unification of Japan under the Tokugawa Shogunate. In 1615, the shogunate issued the Ikkoku Ichijo (One Castle Per Province) decree, which mandated that each domain could maintain only one primary administrative castle. This led to the immediate destruction of thousands of secondary forts and outposts. Today, while only 12 original castle keeps remain from the pre-modern era, the landscape of Japan is still dotted with the ruins, moats, and foundations of the thousands of castles that once defined its feudal history.