Glendalough is renowned for its Early Medieval monastic settlement founded in the 6th century by St Kevin.
His fame as a holy man spread and he attracted numerous followers. He died in about 618. For six centuries afterwards, Glendalough flourished and the Irish Annals contain references to the deaths of abbots and raids on the settlement.
The destruction of the settlement by English forces in 1398 left it a ruin but it continued as a church of local importance and a place of pilgrimage. The buildings which survive probably date from between the 10th and 12th centuries.
The Gateway to the monastic city of Glendalough is one of the most important monuments, now totally unique in Ireland. It was originally two-storied with two fine, granite arches. The antae or projecting walls at each end suggest that it had a timber roof. Inside the gateway, in the west wall, is a cross-inscribed stone. This denoted sanctuary, the boundary of the area of refuge. The paving of the causeway in the monastic city is still preserved in part but very little remains of the enclosure wall.
The Round Tower built of mica-slate interspersed with granite is about 30 metres high, with an entrance 3.5 metres from the base. The conical roof was rebuilt in 1876 using the original stones. The tower originally had six timber floors, connected by ladders. The four storeys above entrance level are each lit by a small window; while the top storey has four windows facing the cardinal compass points. Round towers, landmarks for approaching visitors, were built as bell towers, but also served on occasion as store-houses and as places of refuge in times of attack.