 These, it was believed, would be virtually invulnerable to enemy fire.It was originally envisaged that the fort would have two tiers (in a design similar to that of Garrison Point Fort at Sheerness) mounting around 56 guns; 28 in casemates and the rest in barbettes on the fort's roof.Construction began on this basis in July 1861 but as the work progressed the design was changed, leaving the fort with only a single tier of casemates.