The round tower at Glendalough is considered by many to be one of the most finely constructed and beautiful towers in Ireland. Situated in a thickly forested valley, the 30.48 metre tall tower is built of mica schist with a granite doorway. The conical roof was rebuilt in 1876 using the original stones that were found inside the tower. The round tower is divided internally into 6 storeys by timber floors, connected by ladders. The four storeys above entrance level are each lit by a small window; while the top story has four windows facing the cardinal compass points.

In medieval Ireland, round towers had many uses. They served as beacons to guide pilgrims from afar, bell towers (their Irish name is Cloig-theach meaning bell-tower), storehouses, lookouts and places of refuge in times of attack. The door is about 3.5 meters from the ground, which is commonly believed to be a defensive practice with refuge seekers raising the ladder from within. The doorway was constructed with large stones similar to the cyclopean doorway at St Mary's Church. The arch above the door was carved from a single stone. It is still in near perfect condition even though it is almost 1,000 years old.