Following the Anglo-Dutch capture of Gibraltar in 1704 and the subsequent Twelfth Siege of Gibraltar, in which the Spanish and their French allies sought unsuccessfully to recover the territory, the Spanish built a permanent line of fortifications facing south across the isthmus. Construction began in November 1730 under the Marquis of Verboom with the intention that the lines would block any British invasion of Spanish territory mounted from Gibraltar, act as a starting point for any future Spanish operations against Gibraltar, and cut off access to the territory by land. The lines covered a distance of 950 toises () and were built 1,000 toises () from the Rock of Gibraltar, where the British defences began.