The fortress consists of a strong curtain wall having circular towers mounted with cannon at irregular intervals. Most of them have been restored with fresh blue limestone.
The Connaught tower, which stands isolated in the courtyard, is considered the oldest part of the fortress, and usually supposed to have formed the keep of the first Norman castle built in King John’s reign. It is decagonal in form, but owing to having been pebble-dashed and whitened of late years, it does not retain an appearance of antiquity.
The English stronghold was erected on the site of an old Celtic fortress of the O’Connors. It is recorded that the castle and bridge of Athlone were built in 1129 by Turloch O’Connor, “in the summer of the drought.”
The following year they were demolished by Murogh O’Mleghlin and Feirnan O’Rorke, and in 1153 the castle was burned.
Between 1210 and 1213 the Norman fortress was erected by John de Grey, Bishop of Norwich, in his capacity of Lord Justiciary of Ireland. During its building a tower fell and killed Lord Richard Tuit, who founded the Cistercian Abbey of Granard, County Longford.
Athlone Castle was built on abbey land, and in 1214 King John commanded Henry, Archbishop of Dublin, to give the monks a tenth of the expenses of the castle in lieu of the land used, in accordance with the conditions agreed to by the Bishop of Norwich when he was fortifying it. After this there are several references in the State Documents to the tithes and other compensation due to the monks.
In 1221 the King instituted a fair to be held at the castle.
The fortress being situated on the border of Irish territory, its early history has an exceedingly stormy record. In 1226 Geoffrey de Marisco, who was then Justiciary of Ireland, complained that as the King of Connaught refused to come to Dublin, he had appointed to meet him at Athlone, although the castle was fortified against the Crown.
In 1232 an order was issued to Hubert de Burgh to deliver the castle to Peter de Rivall, and the next year one to Richard de Burgh, who was to surrender it to Maurice FitzGerald, Justiciary of Ireland.
Walter de Lacy received twenty marks for the custody of the fortress in 1240, and eleven years later a tax was levied for its repair.