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[12] 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,[13] having a sloping scarp and counterscarp, the former twenty-five feet in height, the latter, ten. [12] In fortification, the salient angle is that which turns from the centre of a place; while the _re-entering_ points directly towards it. [13] A _hornwork_ is a work having a front and two branches. The front comprises a curtain and two half-bastions. It is smaller than a _crown-work_, and generally employed for effecting similar purposes. 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 fieldpieces, 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. Two days passed before the allies could cross the river. On the 19th August the passage was effected, and the French outposts on St. Michael were driven in. That night, the hornwork itself was carried after a sanguinary assault, the British losing in this short and murderous affair upwards of four hundred men. From the hill, now in possession of the allies, it was decided that the future operations should be carried on, and the engineers arranged that each line in succession should be taken by assault. The place, on a close examination, was ascertained to be in no respect formidable; but the means to effect its reduction, by comparison, were feebler still. Nothing, indeed, could be less efficient; three long 18-pounders, and five 24-pound howitzers, formed the entire siege artillery that Lord Wellington could obtain. The headquarters were fixed at Villa Toro. The engineering department intrusted to Colonel Burgoyne, and the charge of the artillery to Colonels Robe and Dickson.