The greatest ruler of Japan's history, Tokugawa Ieyasu, used one castle as his primary residence when the capital of Japan was Kyoto. It was called Nijo-Jo. This 5-minute video offers a detailed explanation of the history and beauty of this spot. A must see for all tourists.
For more information on Nijo-Jo:
Japan guide's Information: https://www.japan-guide.com/e/e3918.html
The official Kyoto Travel Guide of NiJo-Jo: https://kyoto.travel/en/shrine_temple/119
The official website of Nijo-Jo itself: https://www2.city.kyoto.lg.jp/bunshi/nijojo/english/
A helpful tourist webpage from JapanVistor.com: