The Tiber River is a major geographic feature in Roman history. In Rome's legendary period, Horatius defended Rome against the Etruscans who had planned to cross Rome via a wooden bridge across the Tiber. In Rome's imperial period, the Milvian Bridge, built in 109 B.C., on which Constantine had his vision and conversion, also crossed the Tiber. Learn the basics of the Tiber River and the bridges that crossed it.

© Gareth Cattermole/Getty Images