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.