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,