The monastery was founded by St. Patrick in the 5th century and the remnants of the High Crosses and Round Tower are indicative of its importance. The monastery was linked to St Maelruns monastery of Tallaght and adopted the practices of the Ceile De movement. The Vikings raided the monastery in 932 and it was again burned in 1114. It was the threat from the Vikings, which led to the building of the round tower in the 9th/10th century. A town developed around the monastery with buildings of timber and mud which have since been obliterated by wind and weather.The Round Tower at Old Kilcullen is about 10m high and it has a round-headed doorway. This is less than 2m above ground level and about 1.7m high.
Ancient history books such as the Annals of the Four Masters tell of terrible Danish attacks on Kilcullen in 936 and 944 AD. This probably led to the construction of the Round Tower.
The tower is approximately 40 ft high and the top parts suffered a lot of damage in 1798. An account written in 1782 tells of there being four large windows in the upper part of the tower but only the semblance of one now remains. The village which developed around the edge of the monastery continued to exsist until it was distroyed in a battle on 24th May 1798.