Massena had commenced the invasion before these instructions reached him; but to understand his operations it is essential to have a clear idea of the country in which they were conducted. The advanced positions of the allies extended from Almeida over the Sierra de Estrella, by Guarda to Fundao, Sarzedas, and Castello Branco: no enemy could penetrate that line unless by force, and a serious attack on any one point was to be the signal for a gradual retreat of the whole, in concentric directions towards the Lines. But, if Guarda were evacuated, the enemy while menacing Celerico, could move either by Belmonte or Covilhao and separate general Hill from lord Wellington, the distance between those generals being twice as great as the enemy’s perpendicular line of march would be. To balance this disadvantage, the road from Covilhao was broken up, a Portuguese brigade placed in Fundao, and general Leith’s corps was stationed at Thomar, between two entrenched positions, which formed the second temporary line of resistance. The first of those positions was behind the Zezere, extending from the Barca de Codies to the confluence of that river with the Tagus. The second behind the Alva, a strong and swift stream descending from the Estrella and falling into the Mondego some miles above Coimbra. Both were strong, the rivers deep and difficult of access, and the Sierra de Murcella closely hugs the left bank of the Alva. Hill’s line of retreat from Sarzedas to the Zezere, has been already noticed, and from that river to the Alva, there was a military road constructed through the mountains to Espinhal. But the country from Celerico to the Murcella, a distance of about sixty miles, is one long defile, lying between the Sierra Estrella and the Mondego. The ridge upon which Celerico stands, being a shoot from the Estrella, and encircled by a sweep of the Mondego, closes this defile in front. In like manner the Sierra Murcella, covered by the Alva river, closes it in the rear, and the intermediate parts are but a succession of smaller streams and lower ridges. The principal road was repaired and joined to the road of Espinhal, and a branch was also carried across the Mondego to Coimbra. Thus an internal communication was established for the junction of all the corps. Nevertheless, between Celerico and the Alva, the country was not permanently tenable, because, from Guarda and Covilhao, there were roads over the Estrella to Gouvea, Cea, and Gallices, towns in rear of Celerico; and the enemy could also turn the whole tract by moving through Trancoso and Viseu, and so down the right bank of the Mondego to Coimbra.