An emperor is the king or ruler of an Empire. In contrast to a kingdom which is usually monarchical in which a leader rules over a homogeneous people with a single culture, an empire can contain diverse peoples and groups with differing status levels, such as citizens and non-citizens and partial citizens in a hierarchy of privileges. In ancient Rome, when the Republic dissolved after years of constant civil war, the empire was led by a emperors for four hundred years until it fell apart. One of the first emperors was Augustus Caesar who called himself Princeps meaning "first citizen" although he was, in fact, the dictator. Augustus commissioned the poet Virgil to write the master epic The Aeneid.