Throughout the 5th century various invasions conquered the western half of the empire, but at best could only demand tribute from the eastern half. Theodosius II enhanced the walls of Constantinople, leaving the city impenetrable to attacks: it was to be preserved from foreign conquest until 1204.
To spare his part of Empire the invasion of the Huns of Attila, Theodosius gave them subsidies of gold: in this way he favoured those merchants living in Constantinople who traded with the barbarians. His successor Marcian refused to continue to pay the great sum, but Attila had already diverted his attention to the Western Empire and died in 453.
Leo was also the first emperor to receive the crown not from a general or an officer, as in the Roman tradition, but from the hands of the patriarch of Constantinople. This habit became mandatory as time passed, and in the Middle Ages the religious characteristic of the coronation had totally substituted the old form.
In AD 330, Constantine was in firm command of the entire Roman Empire (the first man for a long while to be in that position), and was planning another initiative as significant as his adoption of Christianity.
Falling ill in AD 337, Constantine was at last baptized - only a few days before his death. It has often been asked why he left this necessary act of Christian commitment so late. The answer is probably so as not to waste the magic of baptism, which washes away sins. An emperor can hardly live a blameless life, and there are many blots on Constantine's record - such as his unexplained execution of his eldest son and his second wife in 326.
His sons inherited the parts of the empire which they had already ruled, on behalf of their father, as Caesars. Constantius II, though not the eldest, had the lion's share - Greece, Constantinople and the entire eastern empire. His elder brother, Constantine II, had Spain, Gaul and Britain. The youngest was Constans who controlled Italy and Spain.
Territorially the East Roman, or Byzantine, Empire was at its height at the end of the reign of the Emperor Justinian I (527 - 565 A.D.).
Called "The Great", Justinian presided over an aggressive foreign policy; the chief goal of which was to restore to Imperial rule the lost territories of the Western Roman Empire. Largely due to the extraordinary talents of his two principal generals, Belisarius and Narsus, North Africa, Sicily, Italy, southern Spain were recovered.
Also Emperor John III Comnenus, crowned in 1182, a grandchild of Emperor John II Comnenus, child of Emperor Andronikos I Comnenus (brother of the failed emperor, Manuel I Comnenus), established a clearer line of succession, following primogeniture, after a brief lack of emperor due to a poorly defined line.
John achieved immense success when he married Isabella of Jerusalem and produced more than 10 children with her, though the number is unsure do to his tenancy to sleep with many women, pretending their children to be of his wife and not them, and at least two known miscarriages of the Empress. John V died after only 19 years as emperor, though he took the throne at an older age and had many STDs at his death. Two weeks later, after an attempted regency to overthrow the then oversees Emperor apparent, Niketas Comnenus (second son of John V, after his older brother died at age 15, heir to the throne) took the throne with a Latinised name of Nicolas I of Constantinople in 1281.
The last Byzantine emperor, Constantine XI Paleologus, was last seen entering deep into the fighting of an overwhelmingly outnumbered civilian army, against the invading Ottomans on the ramparts of Constantinople. Mehmed II also conquered Mistra in 1460 and Trebizond in 1461.