The Moorish wall had significant weaknesses which were pointed out as early as 1770, notably the fact that it had no advanced works to protect it from being bombarded or assaulted.

These problems were tackled during the 1840s when Prince Albert's and Wellington Front were built to straighten and strengthen the line of the curtain wall.

900 convicts were involved in its construction while they were housed in a prison ship called HMS Owen Glendower which was docked at Gibraltar Harbour.

It was later estimated that local labour was cheaper, more productive[7] and they could legally work for longer hours than British convicts.

These were replaced in 1878 by a single 12.5-inch rifled muzzle loader (RML) gun weighing 36 tons – one of six brought to Gibraltar in 1877 – that was protected by an iron shield.