But at the period of Scott's invasion of the valley, General Valencia, with the troops that were afterwards convened at Contreras, was stationed at Tezcoco, either for the purpose of observation, or to induce an attack in that quarter, and thus to draw our forces into a snare on the northern route, or to fall on the rear of the American commander if he attacked El Peñon, or advanced by the way of Mexicalzingo. At Santiago Zacualco, west of the lake and on the route, formidable works were thrown up to defend the entire space between the western shore of lake Tezcoco and the mountains; while on the road to Querétaro, at the mountain pass north of Tenepantla, other defences were erected, so as to screen the country on all sides of the group of hills which lies west of the lakes of Tezcoco and San Cristoval and north of the town of Guadalupe Hidalgo.
The fourth and last advance to the city was that which turned to the south from the Hacienda of Buena Vista, and passing by the town of Chalco, led along the narrow land intervening between the shores of lake Chalco