When Emperor Kanmu transferred the capital to Heian-Kyo, he wrote "此国山河襟帯、自然作城" (this country was surrounded by mountains and rivers from which nature formed a castle), recognizing that this country was a castle of nature.
桓武天皇が平安京に遷都したとき、「此国山河襟帯、自然作城」とあるので、この国が自然の城であるという認識があった。