At Rome, sentries were traditionally posted on the Arx to watch for signals displayed on the Janiculum if an enemy approached. A red flag would be raised and a trumpet blown. The Arx was not regularly garrisoned, however, and should not be regarded as a “fort.” However, in the Gallic siege of Rome (387 BC), the Arx was considered the point of last retreat, the capture of which was synonymous with the capture of the city. It thus held a symbolic power beyond its importance in military strategy, and was a central place in archaic Roman religion.
Source: Arx (Roman) – Wikipedia