It was after the Edo period when most castles became flatland castles or low mountain castles due to the Ikkoku Ichijo Rei (Law of One Castle per Province) that ordered to destroy mountain castles throughout the country.
城のほとんどが平山城・平城に移行するのは、一国一城令によって各地の山城を破却する江戸時代以降の事になる。