The name Glendalough (to derive = sich ableiten) derives from the Irish words Gleann (valley) dá (two) locha (lakes) which in English is "the valley of the two lakes".
The Upper Lake, Glendalough
In the 6th century, Kevin (now known as Saint Kevin) founded a monastery here which became one of Ireland's great (geistliche Zentren) ecclesiastical centres. Though often attacked by the Danes, it remained (bedeutend) significant for about 500 years. In 1214, however, Norman invaders destroyed the monastery. As a consequence, Glendalough became (to desert = verlassen (ein Ort wird von allen Einwohnern verlassen)) deserted and (to fall into decay = verfallen) fell into decay.
Today this beautiful place in the Wicklow Mountains is one of the most important sites of (Klosterruinen) monastic ruins in Ireland. You can take a walk around the lakes and visit the ruins of the church, the cathedral, the priest's house and the (Rundturm) Round Tower.
Round Towers are a common feature in Ireland. It's not absolutely clear, however, what they were used for. Some say that they were some kind of bell towers, used to call the monks in for prayer and dinner. Others say that the monks found (Zuflucht) refuge in the towers when being attacked by Viking (Angreifer, Eindringlinge) invaders. This would explain why the door to a Round Tower is several meters above the ground.