From the website:
The Panzer III is Germany's standard tank, making up over two-thirds of their tank strength. It has good mobility, is well armoured, and is armed with an effective 5cm (2-inch) gun. Manned by veteran crews that are confident in their abilities and Germany's eventual victory, they are capable of defeating just about any tank in the world.
As the Allies built bigger and better tanks with more powerful guns, the Panzer III began to lose its advantage. The Germans responded by fitting an additional 20mm plate in front of the driver's plate and on the gun mantlet. This restored its effectiveness, but pushed the tank's weight up, almost to the limits of the chassis.