The Roman Republic was the period of ancient Roman history between the founding of the city of Rome in 509 BCE and its transformation into an empire under Augustus Caesar in 27 BCE. During this time, Rome was governed by a strong central government headed by two consuls who acted as chief executives, a well-developed legal system, and a Senate made up of wealthy landowners who voted on laws. Although it has been traditionally characterized as a republic, some scholars now believe that Rome was an oligarchy because only wealthy men could participate in government; this led to social unrest among the poor populace. The Roman Empire was characterized by a strong central government under an emperor, who ruled with absolute power. The emperor was advised by a Senate, but he made all decisions. The empire was characterized by a complex bureaucracy; its government had many layers, with different offices carrying out specialized tasks. The Roman Empire was very successful in its early years. It conquered new lands and expanded its territory. However, over time the empire became overextended and difficult to govern. It faced problems such as corruption, economic decline, and civil unrest. These problems led to the fall of the Roman Empire in 476 CE. The fall of the Roman Empire was a slow process that lasted several centuries; there were many factors contributing to its decline—the most important being its internal problems, which weakened it and made it vulnerable to invasion.