The Germans had bombed Stalingrad extensively before attacking, this worked against them because it created huge amounts of rubble which gave the Soviets cover and made using Tanks very difficult. Add in that the Soviets had some foresight that the attack was coming and thus prepared massive fortifications and Stalingrad quickly became a fortress city. The Germans were despite all of this able to take the majority of the city, but they were unable to cross the Volga river where the Russian supply base was. The Soviets practiced a number of measures to stop the Germans from using their overwhelming firepower to decimate the Soviet lines (the most popular tactic was engaging the Germans at close quarters preventing them from calling in artillery bombardments for fear of hitting their own men). Also, halfway through the battle many German air units were withdrawn to fight the allies in Africa.

Soruces:

The Second World War by John Keegan

Stalingrad: The Fateful Siege by Antony Beevor

There was a moment in the battle (November 1942) when Soviets got only three thin stripes of the land (less than half a mile wide - Ludnikov's island) on the left bank of the Volga.