The peculiar geography of Mexico City creates a unique problem for attackers. In those days the city was situated on slightly elevated ground and was surrounded by swampy, marshy fields. The only approach to the city was via straight narrow causeways that were raised. Not unlike some of the terrain in France that had to be traveled by attacking infantry in the invasion of Normandy. AN Advance along any of the them put troops into a perfect position to be shot down like ducks in a row. It also put them into the sights of the cross fire from the city gates.
The US Army was situated so that the most direct approach to the city was along the thee causeways that ran to the city from south to north. The two that approached the city from the southwest and west were dominated by a steep 220 foot tall hill less than 2 miles from the HQ of Commanding General Winfield Scott.
The hill was topped by a “neo romantic” castle that had been transformed into Mexican Military Academy. The road to the top was zigzagging trip up a sheer cliff surrounded at its base by a high stone wall. Chapultepec looked like major obstacle the Mexicans would be certain to defend fiercely due to its symbolic significance.
Lee had proven to be a talented recon scout and engineer with a talented eye for terrain and where to place and sight in artillery for General Scott up to this point in the Mexican Campaign and he put his talents to work finding the best way to attack Mexico City.
Lee was sent to find the best way to attack Mexico City from the south. Setting out with two other engineers, Lee managed to get ” within a mile and a quarter of the city” and observed what the Mexicans were doing to improve and strengthen the defenses. Lee was impressed and became uncharacteristically cautious about the possibility of assaulting Chapultepec. However, he was convinced that the city could be taken by a well coordinated attacks from the south.
Lee spent September 9-11ths in continuous recon missions with no rest, looking for the best place to place artillery. In any case on the 11th Scott decided to assault Chapultepec instead, convinced that a well conducted artillery barrage would drive the Mexican soldiers out of the fortress and when it fell, the city would be open from an attack from it’s summit. With American artillery placed on it the hill would dominate the two western approaches to the city only a mile away.
Lee gave every effort to prepare for the attack on Chapultepec for the next 48 hours. He placed four batteries of heavy artillery , working so furious that the first two batteries were able to open fire on the morning of September the 12th, less than 24 hours after Lee received his orders. This was a major feat. Extreme labor and danger were involved in bringing 8 inch howitzers and 16 and 24 pounder guns across the soft marshy ground and sitting them in platforms that had to be dug by hand with the pick and shovel all in full view of the enemy defenders and under constant enemy fire.
Once the guns were in place Lee moved ahead to examine the approaches to Chapultepec more closely.