The Battle of Stalingrad started on the River Don and ended when the Soviets destroyed the Sixth Army’s flanking formations north and south of the city. But the heart of the battle was in the streets of Stalingrad itself. This is where the Russian soldiers and militia held the German on slaught. Where the defenders demonstrated an unexpected aptitude for urban warfare. And where both sides developed new tactics to deal with fighting in built up areas.
Both the Soviets and Germans poured troops into the Battle of Stalingrad. Having said that the aims were different. Hitler wanted to win in the city. Stalin, or more accurate Zhukov, wanted to hold in the city and win elsewhere. So where as the Germans massed troops the Soviets merely fed in sufficient troops to retain a presence on the western bank of the Volga.