The ancient Dun or Fort of Naas consisted of two Motes, the North Mote and the South Mote. The North Mote is ten metres high, has a diameter of almost one hundred metres at the base, and was built in Viking or early Norman times on the site of the Dun of the Kings of Leinster. It is still in a good state of preservation, and is almost certainly the oldest man-made structure in the town. The 18th century house on the summit was used first as a guard room for the jail, and later as a look-out post and heliograph signal station by the British Army, who were stationed on the Curragh during the 19th century.
The South Mote which is now only discernable as a low hillock, was situated on the site of the present Fairgreen, and shows no indisputable trace of fortifications, apart from the stone wall which is all that remains of an eighteenth century military barracks which existed on the site and was attacked during the 1798 rebellion. It was later replaced by the Naas Infantry Barracks on the Newbridge Road in 1810.