This grass-covered masonry structure, measuring roughly 16 metres east to west and 11 metres north to south, was once the principal stronghold of the Costello clan from at least 1336. Some historians suggest it might be even older, possibly the same fortress of Lugha that was razed by O’Conor in 1270, though the evidence remains tantalising rather than conclusive.

The castle’s strategic importance is evident from its turbulent history of changing hands amongst Ireland’s noble families. In 1580, Theobald Dillon wrested control from the Mac Costellos, and by 1635, it had become the property of Lord Viscount Dillon of Castlereagh and Gallen. Today, visitors can still trace the outline of the rectangular bawn that once protected the castle; a defensive wall that originally enclosed an area of 45 metres by 30 metres. The southern portion has suffered from quarrying over the years, but archaeologists have identified at least three possible hut sites within the enclosure, their circular stone foundations still visible as grass-covered spreads.

Despite multiple archaeological excavations conducted between 1997 and 2006, no significant artefacts have been recovered from the site, leaving many questions about daily life at Castlemore unanswered. A small stream runs immediately west of the castle, joining another watercourse about 100 metres south; a reminder of why this spot was chosen for fortification in the first place. About 100 metres east, across a country road, stands an associated church, suggesting this was once a complete medieval settlement. The entire complex has been protected under preservation order since 1979, ensuring these evocative ruins continue to mark the Roscommon landscape.