Portugal's first line of defence, ironically, would be the string of Spanish border fortresses. Built to threaten Portugal, the massive walls of Astorga, Ciudad Rodrigo and Badajoz were now to defend it. All were still held by the Spaniards and could be expected to defy the French for a number of weeks.

Badajoz on the River Guadiana, This fortress was still in Spanish hands in 1810, and it was the strongpoint from which the Spanish Army of Extramadura operated. The French would have had to capture Badajoz before any advance into Portugal south of the Tagus could have been undertaken.

The flying-bridge at Villa Velha. To enable the rapid passage of troops from the Castello Branco region to the Lisbon peninsula, Wellington established a route which went directly south from Castello Branco to the defile at Villa Velha where a flying-bridge carried the troops across the Tagus. The road then continued through Nisa to the lowest permanent bridge across the Tagus at Abrantes. From Abrantes the main road from Badajoz ran directly to Lisbon. To aid lateral communications and to allow the concentration of Wellington's divisions at his chosen defensive position, another road was also constructed. This was known as the 'military road, and ran from Thomar, via Espinal, to the Ponte do Marcella.

Astorga lies so far to the North that it might well have been ignored by the invaders, but Napoleon had ordered that it should be reduced before the campaign began. This fortress, Napoleon reasoned, gave the Spanish Army of Galicia a stronghold on the plains of Leon from which it might operate against the French rear. Because of its dilapidated medieval walls, its capture would not prove difficult.

Ciudad Rodrigo, by contrast, was a regular fortress and, situated as it was on the Portuguese frontier, was a place of undisputed importance. There was no question about the necessity of the French taking Ciudad Rodrigo before any serious attempt upon Portugal could be considered. For its reduction Napoleon provided a great battering train of fifty large calibre guns, with 2,500 artillerymen and sappers in addition to the artillery of the three corps that were to form the invading army.

Badajoz, far to the South in Spanish Extramadura, was the most formidable fortress upon the frontier. It was much larger and stronger than Ciudad Rodrigo, with towering fortifications set on a rugged hill overlooking the whole countryside. Badajoz also had a ring of strong detached forts in dominant positions. The circuit of ground that would have to be taken up by a besieging army was vast, and a wide, fast-flowing river -- the Guadiana -- cut any lines an attacker would occupy in two.

Portugal's second line of defence was her own frontier fortresses of Almeida and Elvas. Both were capable of a prolonged defence and all that Wellington had to do was ensure that they were adequately garrisoned and provisioned. Almeida was placed under the command of the English Colonel William Cox, with 5,000 men consisting of one regiment of regulars and three regiments of militia. Elvas was held by two regiments of regulars and three of militia -- 8,000 men -- under General Leite.

Only four other Portuguese fortresses were placed in a state of defence: Peniche, Valenza, Abrantes and Campo Mayor. Peniche was a very strong, isolated sea-fortress, on a projecting headland in the Atlantic, forty miles north of Lisbon. It was virtually impregnable. 'The isthmus over which the peninsula of Peniche is approached is covered with water at High tides, and from the Line of Works describing a sort of arc, very powerful cross-fires may be established upon every part of,' noted a British staff officer. [15]

If the French were to penetrate as far as the Lisbon peninsula, Peniche would afford an ideal base for allied raids against the rear of the enemy. It would also act as a secure point from where stranded troops and partisans could be carried off by the Royal Navy.

Valenza, on the River Minho in the North of the country above Oporto, was situated at a strategically important point, and its defences, destroyed by Soult in 1809, were fully repaired. This fortress however, would only come into play if the French made their attack from Galicia.

Abrantes is situated on a hill above the North bank of the Tagus upstream from Lisbon. It is at the point where the road from Spain by way of Castello Branco crosses the road from north to south down the Portuguese frontier from Almeida to Elvas. It was the natural point through which troops could be moved north and south along the frontier, with its permanent bridge protected by commanding fortifications. Wellington ordered its old defences to be re-built and strengthened, under the direction of a British Engineer, with new earthworks and redoubts. [16]

It was garrisoned with two militia regiments. The fortifications of Campo Mayor, a small antiquated fortress close to Elvas, were also repaired and the place garrisoned by a militia battalion.

Elvas and Almeida, Peniche and Abrantes were all regular fortresses with large garrisons. There were also other points where Wellington ordered fortifications of a less permanent kind to be thrown up. The first of these was the line of earthwork redoubts near Penacova behind the River Alva (a left bank tributary of the Mondego), where Wellington expected to make his decisive stand against the French. Here an abrupt ridge commanded the important Ponte da Murcella, the only bridge across the Alva, carrying the most direct road from Celorico to Coimbra south of the Mondego valley. The ridge dominating the left bank of the Alva offered an excellent defensive position to dispute the river crossing.

On 31 January 1810, Wellington ordered the 6-pounder and 9-pounder cannon of the British Reserve Artillery to be moved up from Lisbon and sent to the Ponte da Murcella. [17]

The second was a line of redoubts and trenches, virtually a fortified camp, that was built on the east bank of the Zezere. This covered the main Castello to Abrantes road at the point where it debouches into the river.

French Invasion of Portugal Part 1: Wellington's Plans for the Defence of Portugal

Introduction
The Military Geography of Portugal
The Situation in 1810 and Fortresses
The Devastation of the Countryside and Footnotes