Glendalough, Medieval monastery in County Wicklow, Ireland.

Glendalough is an early medieval monastic complex in a valley in County Wicklow, Ireland. Its round tower, built of mica-slate and granite, stands about 30 metres high with an entrance 3.5 metres above the base. Nearby are the granite Gateway, St Peter and St Paul's Cathedral, and St Kevin's Church, with a conical-roofed tower.

St Kevin, a Leinster nobleman, founded the monastery in the 6th century; he died around 618. It was raided by Vikings and in 1176 'plundered by the foreigners', and the dioceses of Glendalough and Dublin were united in 1214. English forces destroyed the settlement in 1398, yet it survived as a local church and place of pilgrimage until the dissolution of the Irish monasteries in 1539.

The name comes from the Gaelic words for valley and two lakes, directly describing the setting. Visitors today can still see traditional Irish stone crosses in the ancient graveyard, visited by pilgrims for centuries.

The site lies about an hour's drive south of Dublin and offers marked paths of varying lengths through the valley. Visitors should bring sturdy footwear, as trails to the upper lake can be rocky and uneven.

The entrance to the tower structure sits well above the ground, as the lower floor has no door and was only reachable by ladder. Inside, wooden levels once led upward, serving as storage and refuge during attacks.