Karatsu is a sea-facing city in northern Saga Prefecture, close enough to Fukuoka for an easy day trip but different in mood. It was once a port looking toward Korea and China, and that old maritime role still shows in the castle, pottery, fishing villages, and seafood culture.
The compact centre has Karatsu Castle, a reconstructed keep with a fine view over the bay. East of town, Niji no Matsubara runs along the coast in a long belt of black pines. Farther out, Nanatsugama cuts sea caves into basalt cliffs, while Nagoya Castle Ruins recalls the huge military staging ground for Toyotomi Hideyoshi's invasions of Korea.
Karatsu also has a softer side: Karatsu ware, squid in Yobuko, and the riotous Karatsu Kunchi floats every November. It is worth more than a transit stop between Fukuoka and Nagasaki if you like coastal Japan with history in the background.