These,—together with the strong citadel, lying near the garita of Belen in the south-western corner of the city,—were the principal external defences still remaining beyond the immediate limits of the capital. The city itself stands on a slight swell between lake Tezcoco and the western edge of the valley, and, throughout its greater extent, is girdled by a ditch or navigable canal extremely difficult to bridge in the face of an enemy, which serves the Mexicans not only as a military defence but for drainage and protection of their customs. Each of the eight strong city gates were protected by works of various character and merit. Outside and within the cross fires of these gates there were other obstacles scarcely less formidable towards the south. The main approaches to the city across the flat lands of the basin are raised on causeways flanked by wide and deep ditches designed for their protection and drainage. These causeways, as well as the minor cross roads which are similarly built, were cut in many places and had their bridges destroyed so as to impede the American's advance and to form an entangling net work; while the adjacent meadows were in this rainy season either filled with water in many places or liable to be immediately flooded by a tropical storm.
With these fields for his theatre, of action, and these defences still in front of him, it was an important and responsible question, whether General Scott should attack Mexico on the west or on the south.
There can be hardly a doubt that the capture of the hill and castle of Chapultepec, before assaulting the city, was imperatively demanded by good generalship. If the capital were taken first, the Mexicans instead of retreating towards Guadalupe and the north, when we attacked and captured from the south, would of course retire to the avoided stronghold of Chapultepec; and, if our slender forces were subsequently obliged to leave the city in order to take the fortress, our sick, wounded and thinned regiments would be left to the mercy of the mob and the leperos. Chapultepec would thus become the nucleus and garrison of the whole Mexican army, and we might be compelled to fight two battles at the same time,—one in the city, and the other at the castle. But, by capturing the castle first, and seizing the road northward beyond it, we possessed all the most important outworks in the lap of the valley, and cut off the retreat of the Mexicans from the city either to the west, to the castle, or towards our rear in the valley. We obtained, moreover, absolute command of two of the most important entrances to the capital, inasmuch as from the eastern foot of the hill of Chapultepec two causeways, and aqueducts raised on lofty arches, di-
