With the frontier garrisons in the northwest suddenly evacuated, the Christian king Alfonso I of Asturias could hardly believe his luck, and set about dispatching Asturian troops to seize the empty forts. With remarkable swiftness and ease the northwest was captured, and the banks of the upper Ebro were raided by Alfonso and permanently lost to al-Andalus. The Asturians devastated several towns and villages on the banks of the Douro River, and carried off local populations from the towns and villages in the Galician-Leonese lowlands back to the mountains, creating an empty buffer zone in the Douro River valley (the "Desert of the Duero") between the Asturias in the north and al-Andalus in the south. This newly emptied frontier would remain in place for the next few centuries. It is alleged that pastoral Berber mountaineers remained behind in the highlands around Astorga and León. These trapped Berber communities were called ""Maragatos"" by the local Christian Leonese (etymology uncertain, possibly from "mauri capti", "captive Moors"). Although eventually converted to Christianity, the "Maragatos" retained their distinctive dress, customs and lifestyle of Berber origin down to the early modern era.