Caiseal Guth-aird (the stone fort of the high voice) is a Norman Motte and Bailey built over an ancient fort. It is said to be the place where Saint Patrick and his Royal Commission met in winter times to codify the ancient Brehon Laws, and the result of their work can be seen in the book, The Seanchus Mór.
The Castleguard Motte on the eastern side of Ardee would appear to erected around the year 1185 when the Ardee district, later to become a barony, was granted away by John Earl of Mortain to one of his Norman followers Gilbert de Peppard. It may have continued in use down to the fifteenth century. The motte is over 9 metres high, is 60 metres wide at its base and over 16 metres wide at the top. The town walls date from the late 14th and 15th century, but only fragments now remain.