Scale armour is an early form of armour consisting of many individual small armour scales (plates) of various shapes attached to each other and to a backing of cloth or leather in overlapping rows. Scale armour was worn by warriors of many different cultures as well as their horses. The material used to make the scales varied (and included bronze, iron, rawhide, leather, cuir bouilli, seeds, horn, pangolin scales and, in ancient China even paper). The variations are primarily the result of material availability. It was the armor used by the Persian Immortal.
According to the show, the scale armor was strong enough to weaken the blunt force of a celtic sword, but the blow still was able to possibly crack or break a rib. While the Immortal theoretically could continue fighting despite the minor injury, the armor lost some of its scales from the impact. This implies that the armor is too light for a prolonged fight, but can still be effective in a single battle.
The Persian Immortals were able to dominate much of the Middle East due to their superior armor, but the hot climate made them focus on light armor, as heavy armor was too expensive in a region with few raw materials and the weight of armor would fatigue soldiers in the heat. This gave the Immortals a disadvantage against greek hoplites, especially the Spartans. Greeks had a cooler climate and better access to bronze, allowing them heavier armor metallurgy. Because of this, greek hoplites could overpower Immortals if the Persians were unable to outmaneuver the hoplites with their chariots and lighter infantry.