The space between Guarda and the Douro, an opening of about thirty miles leading into the valley of the Mondego, remains to be examined. Across this line of invasion, the Agueda, the Coa, and the Pinel, run, in almost parallel directions from the Sierra de Francia and Sierra de Estrella, into the Douro, all having this peculiarity, that as they approach the Douro their channels invariably deepen into profound and gloomy chasms, and there are few bridges. But the principal obstacles were the fortresses of Ciudad Rodrigo and Almeida, both of which it was necessary to take before an invading army could establish a solid base of invasion. After this the lines of the Douro and of the Mondego would be open; if the French adopted the second, they could reach it by Guarda, by Alverca, and by Trancoso, concentrating at Celerico, when they would have to choose between the right and the left bank. If the latter, they must march between the Mondego and the Estrella mountains, until they reached the Alva, a river falling at right angles into the Mondego, and behind which they would find the allied army in a position of surprising strength. If, to avoid that, they marched by the right of the Mondego upon Coimbra, there were other obstacles to be hereafter noticed; but, in either case, the allied forces, having _interior lines of communication_, could, as long as the Belmonte road was sealed, concentrate in time behind the Alva, or in front of Coimbra. Hence it was on the side of the Alemtejo that danger was most to be apprehended; and it behoved general Hill to watch vigilantly and act decisively in opposition to general Reynier; for the latter having necessarily the lead in the movements, might, by skilful evolutions and rapid marches, either join the sixth and eighth corps before Hill was aware of his design, and thus overwhelm the allied divisions on the Mondego, or drawing Hill across the Tagus, furnish an opportunity for a corps from Andalusia to penetrate by the southern bank of that river. In these dispositions the English general had regard only to the enemy’s actual situation, and expecting the invasion in summer; but in the winter season the rivers and torrents being full, and the roads deteriorated, the defence would be different; fewer troops would then suffice to guard the Tagus, and the Zezere, the Sobreira Formosa would be nearly impassable, a greater number of the allied troops, could be collected about Guarda, and a more stubborn resistance made on the northern line. [Sidenote: Lord Wellington’s Correspondence. MSS.]