This remarkably well-preserved six-storey tower house, measuring 11.9 metres north to south and 8.7 metres east to west, has weathered centuries since its documented existence in 1574. Originally held by John Oshagnes, the castle remained in O'Shaughnessy family ownership for over a century until its confiscation in 1697. Now protected as National Monument 206, the tower retains much of its defensive architecture, including two protective bawns (fortified enclosures), with the castle centrally placed within the inner one.

The castle's interior reveals the sophisticated design typical of Irish tower houses. Entry through a pointed arch doorway in the north wall leads visitors into a small lobby, complete with a murder hole overhead; a stark reminder of the building's defensive purpose. From here, a spiral staircase winds upward through floors that each contain a main chamber with a subsidiary room to the north. Stone vaults separate certain levels, whilst the original wooden floors, once supported by corbels still visible in the east and west walls, have long since disappeared. The tower showcases various defensive and domestic features including garderobes accessed through intramural passages, later-added fireplaces that partially obstruct some original windows, and a mix of window styles ranging from narrow defensive slits to elegant ogee-headed openings.

The castle's defensive capabilities are evident in its remaining architectural details. Angled bartizans project from the northwest and southeast corners at second-floor level, whilst a semicircular machicolation guards the southwest corner at the top of the tower. Three corbels on the northeast corner suggest another defensive feature once stood there. The south gable remains intact with its chimney, though its northern counterpart has been lost to time. At the tower's summit, enclosed crenellations survive between the chimney and southwest bartizan, protected by a projecting string course that runs along the top of the walls. Despite centuries of weathering and partial restoration work, Fiddaun Castle remains an exceptional example of a late medieval Irish tower house, offering visitors a tangible connection to the complex political and social landscape of 16th and 17th century Galway.