The were greek citizen soldiers who fought together in a phalanx to make a formidable heavy infantry

Ancient Greek-citizen soldiers who fought together in a phalanx were called Hoplites.

Hoplites were not professional soldiers but they comprised the bulk of ancient Greek armies of the time. They were mostly represented by free citizens such as propertied farmers and artisans who could afford the weapons and armors. Hoplites were primarily armed with shields and spears and often utilized the phalanx formation as an effective war strategy especially when battling with fewer soldiers. 

The illustration below shows an example of a phalanx formation (Source: http://www.ancientgreekbattles.net/Pics/669505_PhalArgos.jpg). 

He gathered information from primary sources.