Vale Castle was originally an Iron Age hill fort that was then used for a medieval castle. From 1776 it appears to have been used for the defence of St Stephens Harbour. During the Napoleonic Wars it was armed with one 24-pdr and two 9-pdr SB guns on garrison carriages. The intention in 1842 was to upgrade these to a total of five heavy guns, four on traversing carriages and one on a garrison carriage.
Vale Castle was still in military used in 1847 when additions were made to the officers quarters then inside the defence walls (TNA MPHH 1/679).
During the German Occupation the site was heavily fortified including machine gun and mortar emplacements, trenches and gun emplacements.