north side of the lakes was the National Road dominated by El Peñon, a steep hill on which was a fort mounting fifty-one guns. On the southern side of the lakes there was a field of lava considered impassable by troops, in addition to fortified positions at San Antonio, San Angelo, and Churubusco, with an intrenched camp at Contreras, which commanded the southern causeways and must be passed before they could be reached. Beyond these positions were the castle of Chapultepec and the strong inclosure of Molino del Rey protecting the approach to the city gates. General Santa Anna had disposed his troops so that all the positions were strongly garrisoned, and his men outnumbered the Americans nearly three to one.
The engineers reported that the fortress on El Peñon would cost the loss of a third of the army to effect its capture. Reluctant to make this sacrifice, Scott decided to move to the south of the lakes, and, accordingly, sent General Worth in advance as far as San Augustin, about nine miles from the city, where lay the pedregal, a large field of lava which the Mexicans had considered impassable by the Americans. Two miles further on lay the fortified position of San Antonio, which commanded the route between the lake and the lava field, and about three miles distant was the entrenched camp of Contreras, held by General Valencia with a strong garrison and a heavy battery. Scott determined to attack on both sides, and sent Worth forward on the east while Pillow and Twiggs went to the west. The latter advanced over the fields of lava and thus astonished the Mexicans; the way was difficult for infantry and next to impossible for artillery, but Twiggs managed to get a couple of light batteries in position by two o'clock in the afternoon of the 9th, with which he opened fire on the camp. His movement was supported by General Persifer Smith, who endeavored to turn the Mexican left, and so well was the plan carried out that Santa Anna, who was advancing to the relief of General
