If the slightly dubious calculations of ancient historians are to be believed, then the Roman Empire lasted 2,100 years from the days of semi-legendary founders Romulus and Remus. Its final end came in 1453 at the hands of the rising Ottoman Empire, and a Sultan who would afterwards style himself Qayser-i-Rûm: Caesar of the Romans.
The Byzantine Empire
By the age of the renaissance the last remnants of the old Roman Empire were on the final stretch of a millennium of steady decline. Rome itself had fallen in 476, and despite the odd resurgence from the remaining eastern half of the old Empire (known as the Byzantine Empire by some scholars) by the high Middle Ages Roman territory was largely confined to the area around modern Greece and the ancient capital of Constantinople.