As castles developed up until the early modern period from being simply military installations to serving as political centers of government, defensive lines were constructed further outside to take over the role of those functional components (inner castle) that had previously served to defend the castle town and retainers.
近世にいたり、城郭が単なる軍事拠点のみならず政治的統治拠点としての役割を持つようになると、城下町や家臣団防備の目的で従来の城の機能的構成部分（内郭）から、さらにもう一重外側に防御線が設けられるようになった。