Fidenae was an ancient town of Latium, situated about 8 km north of Rome on the Via Salaria, which ran between Rome and the Tiber. Its inhabitants were known as Fidenates. As the Tiber was the border between Etruria and Latium, the left-bank settlement of Fidenae represented an extension of Etruscan presence into Latium. The site of the arx of the ancient town was probably on the hill on which lies the contemporary Villa Spada, though no traces of early buildings or defences are to be seen; pre-Roman tombs are in the cliffs to the north. The later village lay at the foot of the hill on the eastern edge of the high-road, and its curia, with a dedicatory inscription to Marcus Aurelius by the Senatus Fidenatium, was excavated in 1889. Remains of other buildings may also be seen.
Originally a settlement of Etruscans, it was for some while the frontier of the Roman territory and from time to time changed hands between Rome and Veii.
In the 8th century BC during the reign of Rome's first king, Romulus, the Fidenates and the Veientes were defeated in a war with Rome, according to legend. It may be that a colony was established there after the defeat as Livy afterwards describes Fidenae as a Roman colony.
Fidenae and Veii were again defeated by Rome in the 7th century BC during the reign of Rome's third king Tullus Hostilius.
In the early Roman republic Fidenae made a decision that was to cost them much of their land in favor of the new Claudia gens, formed from Sabine defectors. Lucius Tarquinius Superbus, last king of Rome, having been expelled from it, at first looked for intervention from the Etruscans. Lars Porsenna of Clusium, dissatisfied with Superbus' conduct and ethics, made peace with the new republic.