Kuroda Nagamasa, the first lord of Fukuoka province, started constructing Fukuoka castle in 1601 and took 7 years to finish. The castle was a flat-land type and had around 50 observatory towers of various sizes. Among them, Tamon-yagura (designated as an important cultural property), Shiomi-yagura, Otemon, Kinen-yagura, Najima-mon are conserved today. The Wild Cyperus ohwii Kukenth is designated as a natural treasure by the prefectural government and grow in the moats around the castle ruin. There is also a tablet of Manyo poems on the castle site. The castle ruins are designated as a national historic site and it is also called “Maizuru-jo (castle)”.