April 1945. Hitler is in the centre of Berlin, 30 feet (10 meters) underground, surrounded by 13-foot (4m) thick concrete walls, safe from any attack the Allies can throw at him from the air. But the Russians are advancing on the ground. The Red Army is lined up on the Oder River, and they’re coming for the Führer. Blocking the way are rings of ingenious defences, kill zones and strong-points that have turned Berlin into a fortress. At the heart of this fortress is the Führerbunker.