The Gunpowder Mills were of strategic importance being so near to Cork Harbour, and they attracted the attention of the British after the 1798 Rebellion. In 1804, Leslie sold the mills on a lease of 999 years to the British Board of Ordnance. To meet the demands of the British Army, during the Napoleonic Wars, the mill site was expanded tenfold and twelve new mills were added to the complex as well as new processing buildings and homes for the workers and senior officials. To improve security, a barracks was constructed in 1810, and military escorts were arranged to accompany the wagons of powder to Cork Harbour. The site covered , and was enclosed by a high limestone wall.