In antiquity, peoples of the Iranian plateau lived right between the civilized Middle East and the nomads of the steppe.
After the rise of the Scythians they had to find a way to protect themselves from the rapid attacks of the horse archers.
Lacking powerful infantry, they found it in heavy cavalry, armorered so well they were almost invulnerable to arrows, yet fast enough to chase after the enemy.
The Iranians breeded horses that were larger and stronger than the small horses that were the norm at the time.
They key in this revolution was alfalfa, which grew well in the highlands, could feed large numbers and most importantly bigger horses:
the Nisean chargers, the largest of which stood maybe 18 hands tall.
Cataphracts were an elite force, the men recruited from the nobility.
They were not part of a standing army, but could muster quickly against the threat of an attack.
With cataphracts, both rider and horse were armored, starting with scale armor,
later with mail.
The primary weapon of the cataphracts was the lance.
Due to the saddle not being fully developed yet cataphract charges were not as powerful as those of the Medieval knights, but strong nonetheless.
Some riders specialized in the charge, concentrating most armor on the front and leaving the sides and rear somewhat exposed.
Secondary weapons were swords, maces and even bows.
Like in later periods, disciplined heavy infantry could well repel a cataphract charge.
But once softened up by attacks by archers on foot and horseback, they became vulnerable and then the cataphracts could charge to break their formation.
The concept of heavy cavalry was quite successful and neighboring peoples like the Armenians, Byzantines, Tibetans and Chinese also started to use it.
The late Roman army adopted cataphracts and the Byzantines maintained them well into the Middle Ages,
until they were superseded by knights clad in heavier armor.
War Matrix - Cataphract
Iron Age 1100 BCE - 550 BCE, Weapons and technology