That night Clausel abandoned Valladolid, and after crossing the Pisuerga, destroyed the bridge of Berecal. Anxious to unite with Castanos, Wellington waited for the Gallician army to come up--while Clausel leisurely retreated through the valleys of Arlanzan and Pisuerga, as remarkable for beauty and fertility, as for the endless succession of strong posts which they afforded to a retiring army. Clausel, after an able retreat, took a position at Cellada del Camino-- and to cover Burgos, offered battle to the allied commander. The challenge was promptly accepted; but the French general, discovering that a junction of twelve thousand Spaniards had strongly reinforced his antagonist, prudently declined a combat, retired, and united his own to Souham’s corps, which numbered above eight thousand men. This reserve had been organized by Napoleon’s special orders--and was intended to remedy any discomfiture which might befall Marmont in the event of his being defeated by the allies. The British entered the city of Burgos, from which the French had previously retired, after garrisoning the castle with two thousand five hundred men, under the command of General Dubreton. Twelve thousand allied troops, comprising the first and sixth British divisions, with two Portuguese brigades, sat down before the place; while the remainder of Lord Wellington’s army, amounting to twenty-five thousand effective troops, formed the covering army of the siege. The castle of Burgos was a weak fortress, on which French ingenuity had done wonders in rendering it defensible at all. It stood on a bold and rocky height, and was surrounded by three distinct lines, each placed within the other, and variously defended. The lower and exterior line consisted of the ancient wall that embraced the bottom of the hill, and which Caffarelli had strengthened by the addition of a modern parapet, with salient[202] and re-entering flanks. The second was a field retrenchment, strongly palisaded. The third, a work of like construction, having two elevated points, on one of which the ancient keep of the castle stood, and on the other, a well-intrenched building called the White Church; and that being the most commanding point, it was provided with a casemated work, and named in honour of Napoleon. This battery domineered all around, excepting on its northern face, where the hill of St. Michael rising nearly to a level with the fortress, was defended by an extensive hornwork,[203] having a sloping scarp and counterscarp, the former twenty-five feet in height, the latter, ten. Although in an unfinished state, and merely palisaded, it was under the fire of the castle and the Napoleon battery. The guns, already mounted, comprised nine heavy cannon, eleven field-pieces, and six mortars and howitzers; and, as the reserve artillery and stores of the army of Portugal were deposited in the castle of Burgos, General Dubreton had the power of increasing his armament to any extent he thought fit.