Athlone Castle endured its greatest test at the end of the 17th century. Ireland had become a theatre of war for European and religious politics in the 1690s. The unpopular Catholic King of England, James II, was overthrown by a union of English Protestant parliamentarians. They invited his Dutch son-in-law, William of Orange to become King of England, Scotland and Ireland. William crossed to England in 1688, and he quickly gained large support across the land. James fled to France, where he was received into the court of King Louis XIV of France and who also supplied James with military and financial aid. The supporters of King James, known as Jacobites, raised a large army in Ireland. Irish Catholics flocked to the cause in the hope that a victory for James might reverse all the misfortunes of the Cromwellian Settlement. James set sail for Ireland with a well-supplied fleet, accompanied by experienced French officers to help him defeat the supporters of William, known as Williamites.
After their victory at the Battle of the Boyne in 1690, the Williamites effectively controlled the eastern half of Ireland. The River Shannon now became the key frontier, as the Jacobites held the vital strategic castles of Athlone and Limerick. In mid-July 1690, a 7,500 strong Williamite force, commanded by the Scottish General James Douglas, attacked Athlone.
Athlone Castle was defended by 2,000 men, led by the experienced veteran, Colonel Richard Grace. He ordered the abandonment of the eastern part of Athlone Town, and withdrew his forces over the Shannon, breaking down the bridge behind him. When he was summoned to parley terms for surrender, he defiantly fired his pistol in the air and declared that was the only negotiation he wanted. The Williamites lacked the necessary heavy siege artillery, and their field guns made little impression on the strong walls of the castle. Knowing that it would result in many casualties, General Douglas decided against trying to cross the river, and once he discovered that the defenders were to be reinforced, he lifted the siege and withdrew. The gallant defence of Athlone by Grace and his garrison, preserved the line and enabled the Jacobites to continue fighting the war for another year. For Athlone, the greatest test was yet to come.