By this time, Grant, having been engaged in his fifth battle, was almost a veteran; and in this daring warfare was the training process carried on in his mind; in this arena of brilliant strategy was his military education perfected, and his experience enriched by an observation vouchsafed to only a few. The army, having beaten down the obstacle at Cerro Gordo, continued on its march towards the capital, occupying Jalapa and Castle Perote on its way. At Puebla the little force was so reduced by sickness, death, and the expiration of the term of volunteer enlistments, that the veteran general no longer deemed it prudent to advance. His numbers had dwindled down to five thousand; and he rested here, in the heart of Mexico, with his handful of men, for three months, until reënforcements swelled his army to eleven thousand--an insignificant force for the conquest of the country. About the middle of August this little army reached the vicinity of the capital. The city of Mexico is situated on Lake Tezcuco, and is approached over impassable marshes and lagoons by long causeways and bridges. The nature of the country was favorable to an effective defence of the place, especially as the Mexicans had at least four times as many troops in service as their invaders. Located outside of these causeways, and guarding the approaches of the city, were the strongholds of Chapultepec and Churubusco, and the heavy batteries of San Antonio and Contreras, all of them mounting about a hundred pieces of artillery. They were surrounded by morasses, by headlong steeps and rocks heaved into fantastic irregularities by volcanic action. The Mexicans confidently relied upon these natural additions to the strength of their works, and regarded their positions as impregnable. Turning aside from the national road, by which he had marched to Ayolata, and which here presented too many difficulties for the remaining fifteen miles of his journey, Scott made a detour around Lake Chalco, and approached the city from the south. On the 20th of August the battery on the height of Contreras was captured by an impetuous assault, which occupied but seventeen minutes. The garrison of San Antonio evacuated their position, being cut off from the line of defence by the fall of the supporting works. Four miles nearer to the city of Mexico, and commanding the road, frowned upon the invaders the strong fortification of Churubusco, where the main body of the enemy's army had been concentrated for an obstinate resistance; but on the same day the stronghold was battered down, and the Mexicans were driven to their only remaining fortress of Chapultepec. All day long Scott's gallant army had been fighting three times their own numbers, lodged in what had been deemed impenetrable works; but their arms were victorious at every point.