Eastern Roman Empire
The Eastern Roman Empire, also known as the Byzantine Empire, was formed sometime before 540 when the Roman Empire split into separate western and eastern empires. The Eastern Roman Empire became the sole successor of Rome when the western empire collapsed (AUDIO: The Secret History) The city of Constantinople was the capital of the Eastern Roman Empire until the 15th century, when it was captured by the Turks. (PROSE: Shadowmind)
The First Doctor, Vicki Pallister and Steven Taylor visited the Empire in 540. However, the Monk replaced the Doctor with his fifth incarnation, changing the events which originally occurred during their visit. The Fifth Doctor was able to revert the change and his earlier self was reinstated into his rightful place in the timeline. (AUDIO: The Secret History)