Banagher was always a place of great strategic importance because the Shannon and its lowlands provided a natural barrier between Connacht and Leinster. Any army that wanted to cross the river had limited options as apart from Banagher the only other suitable places were Athlone, Shannonbridge and Portumna. This importance was early appreciated by the English also, whose forces seized the town in about the middle of the 16th century coming up the river in order to do so. The fortification known as Cromwell’s Castle was originally a medieval structure but its current form dates from the mid-seventeenth century – the garrison town had been overrun by the forces of the Confederate Catholics in 1642 but was recaptured by Cromwell’s army in 1650. The Cromwellians then established a new fortification on the Connacht side of the river in preparation for the plantation of Connacht in 1654. As a result, this fortification became known as Cromwell’s Castle. As part of the general upgrade of defensive structures in response to the threat of a Napoleonic invasion it was converted for use as a battery. The castle was modified further in 1817 to enable it to mount artillery with a platform for a 24-pound traversing gun constructed on its roof. The interior became a powder magazine and housed a garrison of approximately 20 soldiers.
Having fallen into a state of disrepair the structure came under the care of the Banagher Branch of the Offaly Historical Society in the 1980s and considerable restoration work has been undertaken since then, and continues to be carried out.