Toby, Ronald P. (1991). State and Diplomacy in Early Modern Japan: Asia and the development of the Tokugawa bakufu, pp. 45–46, citing manuscripts at the Historiographical Institute of the University of Tokyo: "Ieyasu granted the Shimazu clan the right to "rule" over Ryukyu... [and] contemporary Japanese even referred to the Shimazu clan as 'lords of four provinces', which could only mean that they were including the Ryukyu Kingdom in their calculations. However, this does not mean that Ryūkyū ceased to be a foreign country or that relations between Naha and Edo ceased thereby to be foreign relations."
Okinawa Prefectural reserve cultural assets center (2015). "首里城跡". sitereports.nabunken.go.jp. Okinawa Prefectural Board of Education. Retrieved September 2, 2016.
Okinawa Prefectural reserve cultural assets center (2015). "沖縄県の戦争遺跡". Comprehensive Database of Archaeological Site Reports in Japan. Retrieved September 2, 2016.