Soviet forces had to fight room by room; while defending the Red October Steel Factory during the Battle of Stalingrad, and in 1945, during the race to capture the Reichstag; despite heavy bombardment with artillery at point blank range (including 203 mm howitzers). It is also difficult to destroy underground or heavily fortified structures such as bunkers and utility tunnels; during the Battle of Budapest in 1944 fighting broke out in the sewers, as both Axis and Soviet troops used them for troop movements. The characteristics of an average city include tall buildings, narrow alleys, sewage tunnels and possibly a subway system. Defenders may have the advantage of detailed local knowledge of the area, right down to the layout inside of buildings and means of travel not shown on maps. The buildings can provide excellent sniping posts while alleys and rubble-filled streets are ideal for planting booby traps. Defenders can move from one part of the city to another undetected using tunnels and spring ambushes. Meanwhile, the attackers tend to become more exposed than the defender as they must use the open streets more often, unfamiliar with the defenders' secret and hidden routes. During a house to house search the attacker is often also exposed on the streets. The Battle of Monterrey was the US Army's first major encounter with urban warfare. It occurred in September 1846 when the US Army under Zachary Taylor invaded the town. The US Army had no prior training in urban warfare and the Mexican defenders hid on rooftops, shot through loopholes, and stationed cannons in the middle of the city's streets. The houses at Monterrey were made of thick adobe, with strong double doors and few windows. The rooftops were lined with a two-foot-tall wall that acted as a parapet for the defending soldiers. Each home was a fort unto itself. On September 21, 1846, the US Army which included some of its best soldiers, recent West Point graduates, marched down the city's streets and were cut down by the Mexican defenders. They could not see the men hidden behind walls, loopholes, or rooftops. They tried to march straight down the street until the intense fire drove them to hide in adjacent buildings. Taylor tried to move artillery into the city but it could not hit the well-hidden defenders any better than the US soldiers could. Two days later the US again assaulted the city from two sides and this time they fought differently. Not wanting to repeat the mistakes of the 21st, General William Jenkins Worth listened to his Texan advisers.