The Lines of Torres Vedras were lines of forts and other military defences built in secrecy to defend Lisbon during the Peninsular War. Named after the nearby town of Torres Vedras, they were ordered by Arthur Wellesley, Viscount Wellington, constructed by Colonel Richard Fletcher and his Portuguese workers between November 1809 and September 1810, and used to stop Marshal Masséna's 1810 offensive. The Lines were declared a National Heritage by the Portuguese Government in March 2019.

At the beginning of the Peninsular War (1807–14) France and Spain signed the Treaty of Fontainebleau in October 1807. This provided for the invasion and subsequent division of Portuguese territory into three kingdoms. Subsequently, French troops under the command of General Junot entered Portugal, which requested support from the British. In July 1808 troops commanded by Sir Arthur Wellesley, the later Duke of Wellington, landed in Portugal and defeated French troops at the Battles of Roliça and Vimeiro. This forced Junot to negotiate the Convention of Cintra, which led to the evacuation of the French army from Portugal. In March 1809, Marshal Soult led a new French expedition that advanced south to the city of Porto before being repulsed by Portuguese-British troops and forced to withdraw. After this retreat, Wellesley's forces advanced into Spain to join 33,000 Spanish troops under General Cuesta. At Talavera, some 120 kilometres (75 mi) southwest of Madrid, they encountered and defeated 46,000 French soldiers under Marshal Claude Victor. After the Battle of Talavera, Wellington realised that he was seriously outnumbered by the French army, giving rise to the possibility that he could be forced to retreat to Portugal and possibly evacuate. He decided to strengthen the proposed evacuation area around the Fort of São Julião da Barra on the estuary of the River Tagus, near Lisbon.

In October 1809, Wellington, drawing on topographical maps prepared by José Maria das Neves Costa, and making use of a report that was prepared for General Junot in 1807, surveyed the area north of Lisbon with Lieutenant-Colonel Richard Fletcher. Eventually they chose the terrain from Torres Vedras to Lisbon because of its mountainous characteristics. From north to south, great undulations created peaks that straddled deep valleys, great gullies and wide ravines. The rugged and inhospitable area offered numerous possibilities for a stubborn rearguard fight from forts on many of the peaks.

Following the decision on the location, Lieutenant-Colonel Richard Fletcher ordered the work to begin on a network of interlocking fortifications, redoubts, escarpments, dams that flooded large areas, and other defences. Roads were also built to enable troops to move rapidly between forts. The work was supervised by Fletcher, assisted by Major John Thomas Jones, and 11 other British Officers, four Portuguese Army Engineers, and two KGL officers. The cost was less than £200,000 according to the Royal Engineers, one of the least expensive but most productive military investments in history.

When the results of the surveys by the Royal Engineers were completed, it was possible, in February 1810, to begin work on 150 smaller interlinking defensive positions, using, wherever possible, the natural features of the landscape. The work was accelerated after the loss to the French of the fortress at the Siege of Almeida in August 1810, which prompted the public conscription of Portuguese labourers to work on the lines. The works were sufficiently complete to halt the advance of the French troops, who arrived in October of the same year. Even after the French had retreated from Portugal, construction of the lines continued in expectation of their return, and in 1812 34,000 men were still working on them. On completion there were 152 fortifications with a total of 648 cannon.

The work began on the main defensive works on 3 November 1809, initially at the Fort of São Julião da Barra and almost immediately afterwards at the Fort of São Vicente (St. Vincent) overlooking the town of Torres Vedras and at the Fort of Alqueidão on top of Monte Agraço. The entire construction was carried out in great secrecy and the French never became aware of it. Only one report appeared in the London newspapers, a major source of information for Napoleon. It is said that the British government did not know about the forts and was stunned when Wellington first said in dispatches that he had retreated to them. Even the British Ambassador in Lisbon appears to have been unaware of what was happening. These defences were accompanied by a scorched earth policy to their north in which the inhabitants were told to leave their farms, destroying all food they could not take and anything else that might be useful to the French. Although ultimately contributing to the success of the defence, this policy led to high rates of mortality among the Portuguese who had retreated south of the lines. By some estimates 40,000 died.

Labour for construction of the forts was supplied by Portuguese regiments from Lisbon, by hired Portuguese and, ultimately, through conscription of the whole district. The 152 works were supervised by just 18 engineers. The Lines were not continuous, as in the case of a defensive wall, but consisted of a series of mutually supporting forts and other defences that both guarded roads that the French could take and also covered each other’s flanks. The majority of the defences were redoubts holding 200 to 300 troops and three to six cannon, normally 12-pounders, which could fire canister shot or cannonballs. Each redoubt was protected by a ditch or dry moat, with parapets, and was palisaded. By the time the French reached the First Line in October 1810, 126 works had been completed and were manned by 29,750 men with 247 heavy guns. Wellington did not use his front-line troops to man the forts: instead, manpower was mainly provided by the Portuguese. Construction continued after the withdrawal of the French and was not fully completed until 1812.

Originally the Second Line was intended to be the main line of defence, 30 km (19 mi) north of Lisbon. The First Line, or Outer Line, was approximately 10 km (6.2 mi) to north of the Second Line. The original purpose of the First Line was to only delay the French. In fact, the First Line was not the original plan, the work was only carried out because the defenders were given extra time due to the slow advance of the French Army. In the end, the First Line succeeded in holding the French and the Second Line was never required. A Third Line, surrounding the Fort of São Julião da Barra near Lisbon, was built to protect Wellington’s evacuation by sea from the fort. A fourth line, of which little remains, was built south of the Tagus opposite Lisbon to prevent a French invasion of the city by boat.