Another siege was mounted in 1727 but the Spanish failed to retake Gibraltar, as the British were once again able to reinforce and resupply the garrison by sea. Following the siege, the Spanish began the construction in 1730 of the Lines of Contravallation, a fortified structure across the entire width of the isthmus anchored by two major forts on each end. This was intended to block access from Gibraltar to the Spanish mainland, and also to serve as a base for any future sieges. The territory's importance increased following Britain's defeat in the Battle of Minorca in 1756, when a French naval victory led to the surrender of the British garrison there.