Karatsu Castle is a 1960s reconstruction on a hill at the edge of Karatsu Bay. The keep is not original, but the position is excellent, looking over the sea, the town, and the long green stripe of Niji no Matsubara.
Come for the view as much as the exhibits. In spring the castle grounds are especially pretty, but even out of season the bay-side setting makes it Karatsu's natural first stop.