The main geographical, as well as the main political, characteristics of Spain are determined by its very complicated mountain-system. It is a land where the rivers count for little, and the hills for almost everything, in settling military conditions. In most countries great rivers are connecting cords of national life: their waters carry the internal traffic of the realm: the main roads lie along their banks. But in Spain the streams, in spite of their length and size, are useless. They mostly flow in deep-sunk beds, far below the level of the surrounding country-side. Their rapid current is always swirling round rocks, or dashing over sandbanks: often they flow for mile after mile between cliffs from which it is impossible to reach the water’s edge. In the rainy season they are dangerous torrents: in the summer all save the very largest dwindle down into miserable brooks. A river in Spain is always a sundering obstacle, never a line of communication. Only for a few scores of miles near their mouths can any one of them be utilized for navigation: the Douro can be so employed as far as Freneda on the frontier of Portugal, the Tagus in good seasons as far as Abrantes, the Guadalquivir to Seville. For the rest of their long courses they are not available even for the lightest boats.
Spanish rivers, in short, are of importance not as lines of transit, but as obstacles. They form many fine positions for defence, but positions generally rendered dangerous by the fact that a very few days of drought may open many unsuspected fords, where just before there had been deep and impassable water. Rivers as broad as the Tagus below Talavera and the Douro at Toro were occasionally crossed by whole armies in dry weather. It was always hazardous to trust to them as permanent lines of defence.
It is the mountains which really require to be studied in detail from the military point of view. Speaking generally we may describe the Iberian system—as distinct from the Pyrenees—as consisting of one chain running roughly from north to south, so as to separate the old kingdoms of Castile and Aragon, while at right angles to this chain run a number of others, whose general courses are parallel to each other and run from east to west. There is no single name for the mountains which separate Castile and Aragon, nor do they form one continuous range. They are a number of separate systems, often divided from each other by wide gaps, and sometimes broadening out into high tablelands. The central nucleus, from which the rest run out, lies between the provinces of New Castile and Valencia, from Guadalajara in the former to Morella in the latter. Here there is a great ganglion of chaotic sierras, pierced by hardly a single practicable road. Northward, in the direction of Aragon, they sink down into the plain of the Ebro: southward they spread out into the lofty plateau of Murcia, but rise into higher and narrower ranges again as they get near the frontier of Andalusia.
This block of chains and plateaus forms the central watershed of Spain, which throws westward the sources of the Douro, Tagus, Guadiana, and Guadalquivir, and eastward those of the Xucar and Segura. The basins of these streams and their tributaries form three-fourths of the Iberian Peninsula. The rest consists mainly of the great valley of the Ebro: this hardly falls into the system, and is somewhat exceptional. It has been described as serving as a sort of wet-ditch to the main fortification of the peninsula. Starting in the western extension of the Pyrenees, quite close to the Bay of Biscay, it runs diagonally across Spain, more or less parallel to the Pyrenees, and falls into the Mediterranean between Catalonia and Valencia. It is more low-lying than the rest of the main valleys of Spain, is broader, and is not so much cramped and cut up by mountains running down to it at right angles to its course.
Behind the Ebro lie, chain after chain, the parallel sierras which mark off the divisions of the great central plateau of Spain. Arteche compares them to the waves of a great petrified sea, running some higher and some lower, but all washing up into jagged crests, with deep troughs between them.
The first and most northerly of these waves is that which we may call the range of Old Castile, which separates the basin of the Ebro from that of the Douro. At one end it links itself to the Pyrenean chain in the neighbourhood of Santander: at the other it curves round to join the more central sierras in the direction of Soria and Calatayud. It is the lowest of the chains which bound the central plateau of Spain, and is pierced by three practicable roads, of which the most important is that from Vittoria to Burgos.
Between this chain on the east and the Cantabrian mountains on the north lies the great plain of Old Castile and Leon, the heart of the elder Spanish monarchy, in the days when Aragon was still independent and Andalusia remained in the hands of the Moor. It is a fairly productive corn-producing land, studded with ancient cities such as Burgos, Palencia, Valladolid, Toro, Zamora, Salamanca. The Tierra de Campos (land of the plains), as it was called, was the granary of Northern Spain, the most civilized part of the kingdom, and the only one where there existed a fairly complete system of roads. For want of the isolated mountain chains which cut up most provinces of the Iberian Peninsula, it was hard to defend and easy to overrun. If the mountains that divide it from the Ebro valley are once passed, there is no way of stopping the invader till he reaches the border of Asturias, Galicia, or New Castile. The whole plain forms the valley of the Upper Douro and its tributaries, the Adaja, Pisuerga, Esla, Tormes, and the rest. It narrows down towards Portugal, as the mountains of Galicia on the one side and Estremadura on the other throw out their spurs to north and south. Hence the Lower Douro valley, after the Portuguese frontier has been passed, is a defile rather than a plain. Before Oporto and the estuary are reached, there are many places where the mountains on either side come right down to the river’s edge.
The second chain is much more important, and more strongly marked: it divides Old from New Castile, the valley of the Douro from that of the Tagus. In its central and western parts it is really a double range, with two narrow valleys between its chief ridges. These valleys are drained by the Zezere and Alagon, two tributaries of the Tagus which flow parallel for many scores of miles to the broad river which they feed. If we call this great system of mountains the chain of New Castile it is only for convenience’ sake: the Spaniards and Portuguese have no common name for them. In the east they are styled the Sierra de Ayllon; above Madrid they are known as the Guadarrama—a name sometimes extended to the whole chain. When they become double, west of Madrid, the northern chain is the Sierra de Gata, the southern the Sierra de Gredos. Finally in Portugal the extension of the Sierra de Gata is called the Sierra da Estrella, the southern parallel ridge the Sierra do Moradal. The whole system forms a very broad, desolate, and lofty belt of hills between the Tagus and Douro, through which the practicable passes are few and difficult. Those requiring notice are (1) the Somosierra Pass, through which runs the great northern road from Burgos to Madrid: its name is well remembered owing to the extraordinary way in which Napoleon succeeded in forcing it (against all the ordinary rules of war) in the winter of 1808. (2) There is a group of three passes, all within twelve miles of each other, across the Guadarrama, through which there debouch on to Madrid the main roads from North-western Spain—those from (a) Valladolid and Segovia, (b) from Astorga, Tordesillas, and Arevalo, (c) from Salamanca by Avila. After this group of passes there is a long space of impracticable hills, till we come to the chief road from north to south, parallel to the Portuguese frontier: it comes down the valley of the Alagon from Salamanca, by Baños and Plasencia, on to the great Roman bridge of Alcantara, the main passage over the Middle Tagus. This is a bad road through a desolate country, but the exigencies of war caused it to be used continually by the French and English armies, whenever they had to transfer themselves from the valley of the Douro to that of the Tagus. Occasionally they employed a still worse route, a little further west, from Ciudad Rodrigo by Perales to Alcantara. When we get within the Portuguese frontier, we find a road parallel to the last, from Almeida by Guarda to Abrantes, also a difficult route, but like it in perpetual use: usually, when the French marched from Salamanca to Alcantara, Wellington moved in a corresponding way from near Almeida to Abrantes. This road runs along the basin of the Zezere, though not down in the trough of the river, but high up the hillsides above it. Spanish and Portuguese roads, as we shall see, generally avoid the river banks and run along the slopes far above them.
The next great chain across the Peninsula is that which separates the barren and sandy valley of the Upper Tagus from the still more desolate and melancholy plateau of La Mancha, the basin of the Guadiana. Of all the regions of Central Spain, this is the most thinly peopled and uninviting. In the whole valley there are only two towns of any size, Ciudad Real, the capital of La Mancha, and Badajoz, the frontier fortress against Portugal. The mountains north of the Guadiana are called first the Sierra de Toledo, then the Sierra de Guadalupe, lastly on the Portuguese frontier the Sierra de San Mamed. Their peculiarity, as opposed to the other cross-ranges of the Peninsula, is that at their eastern end they do not unite directly with the mountains of Valencia, but leave a broad gap of upland, through which the roads from Madrid to Murcia and Madrid to Valencia take their way. When the Sierra de Toledo once begins roads are very few. There are practically only three—(1) Toledo by San Vincente to Merida, a most break-neck route winding among summits for forty miles; (2) Almaraz by Truxillo to Merida, the main path from Tagus to Guadiana, and the most used, though it is difficult and steep; (3) Alcantara by Albuquerque to Badajoz, a bad military road parallel to the Portuguese frontier, continuing the similar route from Salamanca to Alcantara.
Leaving the barren basin of the Guadiana to proceed southward, we find across our path a range of first-rate importance, the southern boundary of the central plateaux of Spain: dropping down from its crest we are no longer among high uplands, but in the broad low-lying semi-tropical plain of Andalusia, the richest region of Spain. The chain between the fertile valley of the Guadalquivir and the barren plateau of La Mancha is known for the greater part of its course as the Sierra Morena, but in its western section it takes the name of Sierra de Constantino. The passes across it require special notice: the most eastern and the most important is that of Despeña Perros, through which passes the high road from Madrid to Cordova, Seville, and Cadiz. At its southern exit was fought the fight of Baylen, in which the armies of Napoleon received their first great check by the surrender of Dupont and his 20,000 men on July 23, 1808. Higher up the defile lies another historic spot, on which Christian and Moor fought the decisive battle for the mastery of Spain in the early years of the thirteenth century, the well-known fight of Las Navas de Tolosa. The Despeña Perros has two side-passes close to its left and right: the former is that of San Estevan del Puerto: the latter is known as the ‘King’s Gate’ (Puerto del Rey). All these three defiles present tremendous difficulties to an assailant from the north, yet all were carried in a single rush by the armies of Soult and Sebastiani in 1810. The central pass of the Sierra Morena lies ninety miles to the left, and is of much less importance, as it starts from the most arid corner of La Mancha, and does not connect itself with any of the great roads from the north. It leads down on to Cordova from Hinojosa. Again sixty miles to the west three more passes come down on to Seville, the one by Llerena, the second by Monasterio, the third by Fregenal: they lead to Badajoz and Merida. These are easier routes through a less rugged country: they were habitually used by Soult in 1811 and 1812, when, from his Andalusian base at Seville, he used to go north to besiege or to relieve the all-important fortress of Badajoz.