As World War II ended, it became clear that main battle tanks were the future. Early examples, like the M46 Patton or the T-44, proved that heavy and light tanks were slowly rendered obsolete.
After the war, Sweden found itself in need of such a vehicle. The Stridsvagn m/42 was already very old, so the Swedish government was forced to buy M48 Pattons and Centurions which were very expensive.
In 1952, the Landsverk company received an AMX M4 Mle. 49 prototype from France. After thorough inspection, they concluded that the concept was great, so they later bought the blueprints from the French company AMX, along with some other blueprints of the AMX 50 100.
In 1953, Landsverk began working on a tank that was similar to the Mle. 49, but performed better and was lighter. It was equipped according to the cold climate of Scandinavia.
Not long after, in 1955, the company had finally finished the project and built the first ptototype of the new tank, dubbed Stridsvagn m/55. It was equipped with a French 100mm SA47 gun. The armour wasn't particularly thick: just 140mm on the upper front plate, 40 on the hull sides and 25 at the rear. The turret, however, was tough: 185mm at the front, 100 on the sides and 10 at the rear.
After some fine-tuning, the vehicle entered production as the Stridsvagn 57. It was now armed with a 105mm kan 55 L/53 gun that fired APCR, APFSDS and HEAT ammo. The upper frontal hull plate was now angled at around 65 degrees and the back was reduced to 10mm. Equipped with infrared sights and lots of ventilators, it was well suited for Sweden's cold weather.
It successfully served from 1957 to 1989. While it didn't see any action (except for the few Strv 57s that were sent to the Israeli for use during the Six Day War of 1967), it proved to be a great replacement for the Centurion and later the M60 Patton.

The controls are simple: press 2 to activate the engind, WASD to steer, VTOL to turn the turret and Trim to elevate/lower the gun. It's a 105mm gun that fires every 8.75 seconds.