Watchtowers are powerful Supernal symbols that often manifest during an Awakening.
Historically, the Watchtowers have not always been present. In the time before the Sundering, mages realized magic as a celestial fire that they brought to their people, while in the Hellenistic Age, Awakenings were often clad in the symbolism of myths from a mages native culture. Only after the Atlantean Diamond was formed and the tale of Atlantis was spread among the Awakened, the Watchtowers revealed themselves. In Neolithic times, mages confronted an object that in all likelyhood grew later into the Watchtower.
In Atlantean dogma, the Watchtowers were build by the Oracles, mages who managed to ascend alongside the Exarchs. Through the will of the Oracles, the Watchtowers pierce the Lie and the Abyss, reestablishing contact between the Fallen World and the Supernal Realms. Every Watchtower is associated with a Path and an Oracle that guards it against the depredations of the Exarchs. Normally, the Watchtowers call to a mage during their Awakening and it's through them that a mage is able to draw on the supernal energies.
The Seers Awaken to the same Watchtowers as the Pentacle mages, often claiming that their Awakening has been guided by one of the Exarchs in their dreams.
Although there are five official recorded watchtowers, others are rumored to exist. The recently appeared Sixth Watchtower shows that it might be possible that other Watchtowers might form. Others, as the Unknown Tower, are rumored to exist, but not actively calling for initiates. The Tremere claim that there are at least two others: One they call the Sixth Dragon (the Tremere also claim that the watchtowers are manifested dragons from the time before Atlantis), whom they claim to be the original source of vampirism, and the Final Watchtower, who contains the power of the Soul.
The Abyss also has corrupted versions of the Watchtowers, called the Dur-Abzu.