Personal Memoirs of Ulysses s. Grant

The route followed by the army from Puebla to the City of Mexico was over Rio Frio mountain, the road leading over which, at the highest point, is about eleven thousand feet above tide water. The pass through this mountain might have been easily defended, but it was not; and the advanced division reached the summit in three days after leaving Puebla. The City of Mexico lies west of Rio Frio mountain, on a plain backed by another mountain six miles farther west, with others still nearer on the north and south. Between the western base of Rio Frio and the City of Mexico there are three lakes, Chalco and Xochimilco on the left and Texcoco on the right, extending to the east end of the City of Mexico. Chalco and Texcoco are divided by a narrow strip of land over which the direct road to the city runs. Xochimilco is also to the left of the road, but at a considerable distance south of it, and is connected with Lake Chalco by a narrow channel. There is a high rocky mound, called El Peñon, on the right of the road, springing up from the low flat ground dividing the lakes. This mound was strengthened by intrenchments at its base and summit, and rendered a direct attack impracticable.[Map 2]

Scott's army was rapidly concentrated about Ayotla and other points near the eastern end of Lake Chalco. Reconnoissances were made up to within gunshot of El Peñon, while engineers were seeking a route by the south side of Lake Chalco to flank the city, and come upon it from the south and southwest. A way was found around the lake, and by the 18th of August troops were in St. Augustin Tlalpam, a town about eleven miles due south from the plaza of the capital. Between St. Augustin Tlalpam and the city lie the hacienda of San Antonio and the village of Churubusco, and southwest of them is Contreras. All these points, except St. Augustin Tlalpam, were intrenched and strongly garrisoned. Contreras is situated on the side of a mountain, near its base, where volcanic rocks are piled in great confusion, reaching nearly to San Antonio. This made the approach to the city from the south very difficult.