The Battle of Monterrey featured two days of intense urban warfare on September 21 and 23. The city was perfectly suited to an urban warfare defense. It had narrow streets, houses that stood like small forts, and occasional high points where a fort could be constructed with a clear line of fire around the city. The city's homes had thick adobe walls, strong doors and windows, and a small wall along the rooftops that could be used as a parapet. The Mexican Army placed soldiers on these roofs and put cannons in the middle of the key thoroughfares.
On September 21st, the U.S. Army, as well as some U.S. volunteers, entered the city a number of times trying to flank a fort that was located high on a hill on northeast Monterrey, with clear views of that section of the city.
Most of these units attempted to move straight down Monterrey's streets, highly visible to Mexican soldiers on top of the well-guarded roofs and at the windows inside the second story of the stout homes. The 5th Infantry was virtually decimated when it marched boldly across a small canal and down a road that they believed led to their objective. The smoothbore rifles carried by the regular Army could not shoot accurately enough to hit the well-hidden soldiers. U.S. artillery rolled into the streets, but could not penetrate the thick, adobe walled homes.
On September 23, Zachary Taylor's men entered the city from the east and the west. This time they embraced new urban warfare techniques, taught to them by Texan soldiers. The Texans had already fought twice in Mexico-style cities which contained narrow streets, a town square, and thick walled homes with the parapet lining the roof. They had fought the Mexican army first at San Antonio in 1835, where Santa Anna's brother-in-law headquartered himself at the Alamo and scattered men throughout the rest of the city. They fought in a Mexican city again in 1842 at the Battle of Mier. From those battles they learned how to "mouse hole" from house to house instead of marching down open streets toward an objective. They also learned to never approach artillery in a city head-on, but to instead hide inside the houses and shoot the artillerymen who approached the piece. The Texans, as well as the Mississippians led by Jefferson Davis, possessed rifles rather than smoothbores. The rifles contained rifling which spun the bullet as it exited the barrel, making the rifle more accurate than the standard issue smoothbore. Rifles allowed these volunteers to shoot the well-hidden Mexican soldiers, who sometimes only showed their head above a parapet or the shadow of their body behind a rifle hole carved out in a house wall.
Many of the techniques used at Monterrey are still in use today in modern urban warfare, though with much more sophisticated technology. The young US army did not capture the lessons learned at Monterrey in 1846 and many of these techniques had to be re-learned later by the Army.
For additional information on urban warfare at Monterrey, read "A Perfect Gibraltar, the battle for Monterrey, Mexico".