File:Athlone - Norman Castle (2718875972).jpg

At the heart of Athlone, both geographically and historically, is the castle. The ford of Athlone was strategically important, as south of Athlone the Shannon is impassable until Clonmacnoise (where the Esker Riada meets the Shannon), and north is Lough Ree. In 1001 Brian Bóru led his army from Kincora into the town, his fleet sailing up the river via Lough Derg to attend a gathering.

A bridge was built across the river in the 12th century, approximately 100 metres south of the current bridge. To protect this a fort was constructed on the west bank in the town by Turloch Mór Ó Conor. On a number of occasions both the fort and bridge were subject to attacks, and towards the end of the 12th century the Anglo-Normans constructed a motte-and-bailey fortification here. This was superseded by a stone structure built in 1210 by Justiciar John Gray. The 12-sided donjon dates from this time. The rest of the castle was largely destroyed during the Siege of Athlone and subsequently rebuilt and enlarged.

During the wars that racked Ireland in the seventeenth century, Athlone held a vital position, holding the main bridge over the River Shannon into Connacht. In the Irish Confederate Wars (1641–1653), the town was held by Irish Confederate troops until it was taken late in 1650 by Charles Coote, who attacked the town from the west, having crossed into Connacht at Sligo.