Just a little spoiler for the next chapter;)
Early in 1941 when the German army invaded the Soviet Union, their famed panzer armies came across the better tanks of the Red Army, the KV-1 and the T-34, both was vastly superior to the their German counterparts. Although many were lost, they destroyed many German panzers.
Because of this, the worlds most famous tanks, the Tiger and Panther were made, to counter the supposedly subhuman tanks of the Red Army. And they did, and even better tanks were made, such as the King Tiger which was a very heavy tank, together they destroyed many Soviet tanks, often outnumbered greatly.
However, the tide started to turn against the renowned Panzer Divsions, their reliance on quality hardware soon started to fail in battles such as Kursk and Stalingrad, where the massed numbers of Russian tanks, together with mighty machines with huge cannons like the ISU destroyed large numbers of German tanks.
And then by 1945, the German Army was destroyed, and the masses of Soviet Tanks poured into the Reich. 
This was because the German tanks were too expensive to make, too long to produce and broke down easily, compared with the robust and later up gunned T-34 and the successor of the KV-1 tank, the IS-2, which was faster, had similar armor and carried a more powerful bunker destroyer gun than the King Tiger.
But the Germans never learnt their lesson....and still they tried to make even more expensive tanks, even as the quality of their steel and armor became worse during 1945, the Maus was made....
It never saw combat.
With the uncountable numbers of the Soviet Union pouring into Germany, victory was assured. Berlin would fall, Hitler would die a painful death and the millions of men, women and children sacrificed would be avenged as the battle worn flag of the Soviets would fly proudly over the Reichstag soon.
But the proud Panzer armies of the Germans would not give up, and so many battle groups of Tigers, Panthers and King Tigers headed to more easily defensible positions, one of which was the prototype testing facility of Kummersdorf.
However, a veteran Soviet heavy tank regiment was diverted from the Assault on Berlin to capture the facility too.