Why was The Battle of Stalingrad so Deadly

Why was The Battle of Stalingrad so Deadly? The Battle of Stalingrad. Inarguably one of the most merciless and destructive battles throughout history. Resulting in nearly two million total casualties between August 23, 1942, through February 2, 1943, the German siege resulted in a decisive victory for the Soviets… although at the cost of over a million Russian casualties. The numbers of the Battle of Stalingrad seem ridiculously high, on both sides. So, why was the Battle of Stalingrad so deadly?