The fortress was intended for protection in this direction against the invasions of the O’Byrnes and O’Tooles. The site was some three and a half miles north-east of Glendalough, the ancient cathedral city of that diocese. Its natural defences were the bog, on the edge of which it stood, and thick woods that stretched almost from Dublin to Glendalough. Quite close to it ran a stream, which joined the Avonmore about a quarter of a mile lower down, near the present village of Annamoe. It appears to have been a square building, flanked by towers at each corner. The foundations, which still remain, measure some 120 feet each way. They are elevated about 20 feet, and are now covered with grass. The castle was built of rubble stone and excellent mortar, which is shown by the huge blocks of the walls which still lie round the foundations. The Archbishops held courts and exercised jurisdiction here through their officers, and had their own gallows. In 1277-78 the Treasurer’s account for the year contains £60 to John de Saunford for the custody of the new castle of Mackinegan and Castle Keyvin. The stronghold was often used by the Archbishops as a hunting-lodge, the woods around being well stocked with deer. It was also strongly garrisoned. At the beginning of the next century (1308) the Viceroy Wogan marched against the O’Tooles, but was defeated with the loss of several knights. Castle Kevin was captured and the garrison killed, while the towns near were sacked and plundered. Later Piers de Gaveston successfully subdued the rising, and made a thanksgiving offering at the Church of St. Kevin, Glendalough. The following year he built New Castle in the O’Byrnes’ country and repaired Castle Kevin, at the same time cutting a pass through the woods, from it to Glendalough. Thirty years later Alexander de Bickner received royal orders to repair his fortifications at Castle Kevin, so that at this time it was still connected with the See of Dublin, but it subsequently passed into royal keeping. It appears that Henry VIII. by letters patent “made grants to Arte O’Toole and heirs the manor of Castle Kevin and the Farrtree” (hence Vartry) “on conditions they used the English habit, language, education, hostings, aidings, and the like, and that he should keep Castle Kevin in repair as a bulwark against the rebels.” Phelim O’Toole was the representative of the family in 1591 when Hugh Roe O’Donnell escaped from one of the gate towers of Dublin Castle, where he had been confined as a hostage for over three years. O’Toole having visited him in prison, as a friend, during this time, he naturally thought he was safe in seeking shelter at Castle Kevin.