On the Miño River, Tuy and Orense were frontier towns, to populate which, bishoprics were erected. To the south of Oviedo, and almost on a line with the two Galician towns, Astorga, Leon and Burgos were strongly fortified, and formed an imaginary line to the north of which ruled Christian monarchs, and to the south Arab emirs.
Burgos at the same time served as fortress-town against the rival kings of Navarra to the north and east; the latter, on the other hand, fortified the Rioja against Castile[{71}] until at last it fell into the hands of the latter. Then Burgos, no longer a frontier town, grew to be capital of the new-formed kingdom of Castile.
Slowly, but surely, the Arabs moved southwards, followed by the implacable line of Christian fortresses. At one time Valladolid, Palencia, Toro, and Zamora formed this line. When Toledo was conquered it was substituted by Coria, Plasencia, Sigüenza, and, slightly to the north, by Madrid, Avila, Segovia, and Salamanca. At the same time Sigüenza, Segovia, Soria, and Logroño formed another strategic line of fortifications against Aragon, whilst in the west Plasencia, Coria, Toro and Zamora, Tuy, Orense, and Astorga kept the Portuguese from Castilian soil. In the extreme southwest Cuenca, impregnable and highly strategical, looked eastwards and southwards against the Moor, and northwards against the Aragonese.
In all these links of the immense strategical chain which protected Castile from her enemies, the monarchs were cunning enough to erect sees and appoint warrior-bishops. They even donated the new fortress-cities with special privileges or fueros, in virtue[{72}] of which settlers came from all parts of the country to inhabit and constitute the new municipality.
Such—in gigantic strides—is the story of most of Castile's world-famed cities. In each chapter, dates, anecdotes, and more details are given, with a view to enable the reader to become acquainted not only with the ecclesiastical history of cities like Burgos and Valladolid, but also with the causes which produced the growing importance of each see, as well as its decadence within the last few centuries.