Chaves was known to the Romans as Aquae Flaviae (Flavian's Waters). They established a military base here and popularized the town's thermal springs. The impressive 16-arch Roman bridge across the Rio Tâmega, at the southern end of town, dates from the 1st century AD and displays two original Roman milestones. Today Chaves is characterized most by a series of fortifications built during the late Middle Ages, when the city was prone to attack from all quarters. The town lies only 12 km (7 miles) from the Spanish border. Its name means "keys"—whoever controlled Chaves held the keys to the north of the country.