To gain the valley of the Mondego there are three routes. The first passing by Almeida and Celerico, the second by Trancoso and Viseu, the third by Alfayates and Guarda over the high ridges of the Estrella. To gain the valley of the Zezere, the march is by Alfayates, Sabugal, and Belmonte, and whether to the Zezere or the Mondego, these routes, although rugged, are practicable for artillery; but between Guarda and Belmonte some high table-land offers a position where an army could seal the passage on either side of the mountain, except by the Trancoso road. In fact, the position of Guarda may be called the breast-plate of the Estrella. On the side of Coria, an invading army must first force or turn the passages of the Elga and Ponçul rivers, to reach Castello Branco, and that done, proceed to Abrantes by the valley of the Tagus or over the savage mountain of Sobreira Formosa. But the latter is impracticable for heavy artillery, even in summer, the ways broken and tormented by the deep channels of the winter torrents, the country desert, and the positions if defended, nearly impregnable. Nor is the valley of the Tagus to be followed, save by light corps, for the villages are few, the ridges not less steep than those of Sobreira, and the road quite impracticable for artillery of any calibre. Such, and so difficult, being the lines of invasion through Beira, it would seem that a superior enemy might be met with advantage on the threshold of the kingdom; but it is not so. For, first, the defending army must occupy all the positions on this line of ninety miles, while the enemy, posted at Ciudad Rodrigo and Coria, could, in two marches, unite and attack on the centre, or at either extremity, with an overwhelming force. Secondly, the weakness of the Beira frontier consists in this, _the Tagus along its whole course is, from June to December, fordable as low down as Salvatierra, close under the lines_. A march through the Alemtejo and the passage of the river at any place below Abrantes would, therefore, render all the frontier positions useless; and although there were no enemy on the borders of the Alemtejo itself, the march from Ciudad Rodrigo by Perales, Coria, and Alcantara, and thence by the southern bank to the lowest ford in the river, would be little longer than the route by the valley of the Mondego or that of the Zezere. For these reasons _the frontier of Portugal must be always yielded to superior numbers_. Both the conformation of the country, and the actual situation of the French corps, led lord Wellington to expect, that the principal attacks would be by the north of Beira and by the Alemtejo, while an intermediate connecting corps would move by Castello Branco upon Abrantes, and, under this impression, he made the following dispositions. Elvas, Almeida, and Valença, in the first, and Peniché, Abrantes, and Setuval, in the second line of fortresses, were garrisoned with native troops, part regulars, part militia.