Fukuoka Castle in Chuo-ku was once the main fortification of Kuroda Nagamasa who was granted large land holdings in Chikuzen (modern day Fukuoka) for his efforts in the Battle of Sekigahara. The existing Najima Castle was too small to accommodate the new han, and the larger castle was built between 1601-1607. By most accounts, it was considered an amazing castle at the time. Construction was aided by Noguchi Kazunari who did stone masonry work on the castles at Edo and Osaka. The 47,000 square meter castle complex boasted a stunning 47 yagura (turrets), and was naturally fortified by it's seaside location and position along the Naka-gawa.
Today, only a couple yagura remain, but the stone walls are mostly in tact. It is not as impressive as other castles Ive visited, but certainly worth the time if you are in Fukuoka. Being a large city, public transport is quite easy. Hakata Station is the final stop along the Sanyo Shinkansen line (a couple hours from Osaka). From there, only a short subway ride to Ohorikoen will put you right in front of the castle grounds. And as with most castles, it is now a public park... No charge for admission.
Fukuoka Castle. Fukuoka, Kyushu