During the late years of the Roman Empire, the two halves of the Empire were split and essentially ruled by different leaders. While the western part dealt with Visigoths and other incursions into the northern border, the eastern half dealt with constant fighting with the Persians. Between the 500s A.D. and the 700s, much of the fighting between the Persians and the Byzantines was done from horseback. And the armor of the cavalry on both sides became more and more similar over time, coming to look something like this artistic piece:
The armaments of a Byzantine Cataphract, (a name given to the heavy cavalry, literally meaning “armored” from the Greek, Kataphraktos) would include some or all of the following: two bows, a lance, a spear, a sword, a mace, an axe, a sling, full plate armor, full barding armor and in some cases a banner. Because of the wide variety of their armaments, these warriors likely spent much of each year training in the use of each of their weapons. They were the elite fighting force of their time.
The horsemanship of each man was as important as his familiarity with his weapons. The Cataphracts rode in units and were capable of intricate movements to gain advantages on the field, and this was only possible because of expert horsemanship.
The advantage gained by this kind of a fighting force is that the riders could ride in deep (8-10 deep) ranks and punch through an entire rank of enemy infantry without losing all momentum. They were as much a weapon of fear as an effective fighting force. This tactic was emulated through history up to and even slightly beyond the development of gunpowder weapons.
Part of what allowed the extensive use of heavy cavalry was a decision made by Heraclius, Emperor of the Eastern Roman Empire from 610-641 A.D., who ordered that soldiers of Byzantium would be assigned a parcel of land in reward for their service. Because land is very necessary for maintaining horses, this allowed many more of his men to keep and train horses for battle.