The site’s natural defences, with sheer cliffs to the east and steep slopes to the south, made it an ideal location for a fortress when it was first built in the 13th century. While local tradition suggests it may have been constructed on an earlier site belonging to the Eoganacht Glennamnach, the pre-Norman dynasty who ruled this territory, excavations in the 1980s found no clear evidence of early medieval activity here.

The castle’s layout consists of a roughly four-sided walled enclosure with towers at each corner; three circular and one square. At its heart stands a 13th-century hall-keep, a compact two-storey structure with an attic that originally could only be accessed via a first-floor doorway in the north wall. Archaeological excavations between 1982 and 1984 revealed the castle’s complex building history, uncovering four main construction phases and an intricate range of buildings along the western side. The earliest structure was a rectangular gatehouse dating to the 13th century, complete with chambers flanking a central passage and two tiny vaulted prison cells with trapdoor entrances. During these excavations, archaeologists made an intriguing discovery: a Sheela-na-gig figure hidden away in one of the western prison vaults, apparently concealed there deliberately before the 17th century.

The castle underwent significant modifications in the 15th century, when new sections of curtain wall were built and circular corner towers were added to the southeast and northeast corners. A particularly well-preserved garderobe turret near the southwest corner, built during this phase, still stands to its full height, testament to the exceptional quality of medieval masonry and mortar. The castle continued to evolve through the centuries, with a kitchen added in the northwest corner and a bakehouse featuring a large bread oven in the southwest corner during the late 16th and 17th centuries. The oven’s floor was ingeniously paved with old millstones made from heat-resistant sandstone. Though occupied into the 18th century, Glanworth Castle had become a ruin by the 19th century and was subsequently quarried for building stone. Today, as a National Monument in State Ownership, it offers visitors a chance to explore one of Ireland’s most architecturally complex medieval fortresses.