Shimotabaru Castle is a Ryukyuan gusuku — a fortified enclosure — located in the interior of Hateruma Island. The site dates to the period before Hateruma was formally incorporated into the Ryukyu Kingdom, making it one of the more ancient surviving structures in the Yaeyama Islands. The stone walls are typical of Ryukyuan construction: rough coral limestone stacked without mortar.
The ruins are modest and require some navigation through the island's interior tracks to reach by bicycle. There are no explanatory signs in English and the setting is quiet bush rather than a manicured heritage site. That roughness is part of the appeal. Hateruma's remoteness has preserved it from the tourist infrastructure that surrounds more famous Ryukyuan gusuku on Okinawa.