But all the matters of engineering hitherto mentioned were unimportant and merely subsidiary, when placed beside the one great piece of work which formed the keystone of Wellington’s plan for the defence of Portugal. His whole scheme depended on the existence of an impregnable place of refuge, available both for his army, and for the emigrant population of the country-side which he was about to devastate. He must have a line on which the invader could be finally checked and forced to halt and starve. If such a line had not existed, his whole scheme would have been impracticable, and after a lost battle he might have been driven to that hurried embarkation which the ministers in London foresaw and dreaded. But his eye had been fixed upon the ground in front of Lisbon ever since his second landing in the Peninsula in April 1809, and there he thought that the necessary stronghold might be found. A full year before Masséna’s invasion he had informed the British cabinet that though he could not undertake to defend all Portugal, ‘for the whole country is frontier, and it would be difficult to prevent the enemy from entering by some point or other,’ he yet conceived that he might protect the essential part of the realm, the capital, against anything save the most overwhelming odds[200]. The scheme had taken definite shape in his head when, on October 20, 1809, he wrote his famous dispatch to Colonel Fletcher, the commanding engineer at Lisbon, directing him to draw up without delay a scheme for the construction of two successive lines of trenches and redoubts, covering the whole stretch of country from the Atlantic to a point on the estuary of the Tagus twenty miles or more north of the capital. This was, in its essentials, the order for the construction of the lines of Torres Vedras, for though the front designated does not exactly tally with that ultimately taken up, it only differs from it in points of detail. Fletcher is directed to survey a line from the mouth of the Castanheira brook to the mouth of the Zizandre, and another, a few miles behind, from Alhandra on the Tagus by Bucellas and Cabeça de Montechique towards Mafra. These roughly represent the two lines of defence ultimately constructed, though in the end the extreme right flank was drawn back from the Castanheira to the Alhandra stream. Fletcher is told that the works will be on the largest scale: the fortified camp above Torres Vedras is to hold 5,000 men, the works at Cabeça de Montechique alone will require 5,000 workmen to be set to dig at once; great operations, such as the damming up of rivers and the creation of marshes many miles long, are suggested. [200] For these views of Aug. and Sept. 1809, see vol. ii. p. 610.