The fall of the Roman Empire split the East and West; the east became the Byzantine Empire. The Empire began in 306 at the time of Constantine I until the fall of Constantinople to the Turks in The culture of the Byzantine Empire was a continuation of that from the Roman Empire and was complex and refined. It was a large, ethically diverse Christian state, defended by a mobile specialised army. The Empire was circled by enemies but relied less on military strength and more on persuasion. Even when they did fight, which they often did with skill, they were less inclined to destroy their enemies, rather restrain them as they realised that their current enemies could easily be their next allies.
The Empire began to lose much of its territory to the Arabs but what was left was strengthened by reforms- a new administrative unit introduced along with a system of military land grants and hereditary service that ensured an adequate supply of soldiers. The economy of the Byzantine Empire actually became stronger due to its lack of territory as it gave greater freedom to merchants and agricultural labour.
The Byzantine Empire was an autocracy: this means the Emperor maintained complete control over all branches of government, finance and administration, the judiciary and armed forces, as well as having an enormous influence over the Orthodox church and the financial life of the empire. The office of Emperor was never violated as no Byzantine ever seriously thought about any other form of government until the Empire's last days. But there was a very big difference between theories about the Basileus (Ruler of eastern Roman Empire) and the actual position of individual emperors. Byzantium continued with the Roman idea of an elective monarchy (elected rather than hereditary monarch). Emperors were subject to the Empire's laws and even the Emperor Justinian I, an arch- autocrat (ruler with unlimited authority), recognised in his legal codes that the people had in fact simply transferred their complete independence to the Emperor - he did not rule in his own right. Though in theory the emperors held the central, absolute power they were unable to stop the feudalization of the empire and the accumulating wealth of a few powerful families. While these civil disputes were going on new enemies were gaining strength- the Seljuq Turks and the Normans.
Although several families managed to establish ruling dynasties, Byzantium did not fully develop the notion of hereditary rule. Many able and ambitious men (and at least one woman) from very humble beginnings managed to rise to the top over those who had a 'better' claim in terms of their family background. The Byzantine idea that the Emperor was ultimately selected by God also, wrongly, helped successful rebels and people who illegally seized places of another. If you were able to forcefully remove the existing Emperor and rule in his place you obviously had God's approval - otherwise He would never have allowed you to succeed. The civil service was part of a whole distinct and semi-independent element within the Byzantine political culture, along with the provincial and military aristocracy, the Orthodox Church and, the Emperor himself. Entry into the civil service was only for the elite in society. To join candidates had to be educated to the highest level (which was only available in Constantinople and received patronage from someone already in an influential position. Another factor that contributed to joining the civil service was being castrated as a child. This was because eunuchs could not rule the empire and so were thought to be more trustworthy, though there were members of the civil service who were not eunuchs.