Following the problems the Spanish faced with the buildings sinking into the soft wet ground, the British began to construct fortress walls and battlements on higher, more solid ground. In 1770, chief engineer William Green began preparatory work for the construction of Grand Casemates as bombproof barracks on the square's northern flank. However, its construction was not started until after the Great Siege of 1779-1783 and it was finished in 1817 under the governorship of General Sir George Don.