Spartan males were trained at young age to become relentless soldiers. They went through extremely tough military training. To make these young boys tougher, they were exposed to the elements much more than a normal Spartan. In the winter, they wore no shoes and minimal clothes. At just 20 years old, these young men were put into barracks where they ate, slept, and lived for the next 10 years. They were also kept away from women for all these years. This was designed to teach discipline in Spartan soldiers. It was a successful method as the Spartans had one of the best militaries of that time, maybe all time!
A Spartan phalanx.