The first answer is simple: hereditary rule. For most of this period, emperors were not chosen on the basis of their ability or honesty, but simply because they were born in the right family.

What comes to mind when you think about the kind of Roman Emperor who managed to establish an indestructible empire?

The emperor Hadrian was well known for building monuments across the Roman Empire ... all decorated with exquisite mosaics, busts, and sculptures of gods and heroes modeled on the best examples ...

Sculptures of Roman emperors have gone back on display at a Cambridgeshire estate following an absence of "more than 60 years". The busts were on show at Wimpole Hall from "at least the 1770s ...