The number of castles increased greatly from the middle of the Sengoku period, hirayamajiro (castles built on a hill or low mountain surrounded by a plain) or hirajiro (a castle built on flatland) became dominant, and mountain castles decreased because they were useful for defense, but not appropriate as a base for political power.
戦国時代中期から城の数は飛躍的に増大し、平地に臨む丘陵に築いた平山城（ひらやまじろ）や平地そのものに築いた平城（ひらじろ）が主流となり、防御には優れるが政治的支配の拠点としては不向きであった山城は数が減っていく。