Ravenna was a city located in northern Italy which was a part of various kingdoms and empires throughout the 5th and 6th centuries.
At some point, Ravenna replaced Rome as the capital of the newly formed Western Roman Empire. (AUDIO: The Secret History [+]Error: Code 2 - no data stored in variables, cache or SMW.) In 493, when the Fifth Doctor and Tegan Jovanka visited it, it was part of this empire. Later, the city was conquered by the Vandals (PROSE: Goths and Robbers [+]Error: Code 2 - no data stored in variables, cache or SMW.) and eventually became the capital of the Kingdom of the Ostrogoths. (AUDIO: The Secret History [+]Error: Code 2 - no data stored in variables, cache or SMW.)
In 540, the city was conquered by General Flavius Belisarius for the Eastern Roman Empire during the Gothic wars and the reconquest of Italy. In his first incarnation, the Doctor visited Ravenna in 540 with Steven Taylor and Vicki Pallister,, soon before the Byzantines were to reconquer Italy. However, through the machinations of the Monk to sabotage the Doctor's timeline, the First Doctor's place was taken by the Fifth Doctor. This damage would later be repaired. (AUDIO: The Secret History [+]Error: Code 2 - no data stored in variables, cache or SMW.)