At Rome, sentries were traditionally posted on the Arx to watch for signals displayed on the Janiculum if an enemy approached.[1] A red flag would be raised[2] and a trumpet blown.[3] 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
Tim B.
“On the Arx was located the auguraculum, the open space where the augurs conducted the rituals that determined whether the gods approved of whatever undertaking was at hand, public business or military action. This auguraculum was the stone where the elected monarch, during the Roman Kingdom, was seated by the augurs with his face to the south.[4]”