Each of the 23 Sacred Sanctuary Cities was built by the Greywalkers by hand without using magick of any kind. The only magick inherent in the construction of the Cities was that used to weave the wards, Shields, and Barriers that naturally respond to danger, was the magick that made/makes each City a sentient being all on its own.
The Greywalkers placed six Watchtowers (or obelisks) throughout each City, tethered to its main Watchtower, the Heart, for a total of seven per City. However, the Watchtower of the Qishir, was never placed in the same location in each City as a security measure. For if one destroys that Watchtower, the City will die and with its death its citizens are unprotected from any and all attacks.
Each Watchtower was designed to tether to the Qishir and each member of their Oathed Court, specifically the Triad and Sacred Three. This way, if the walls were somehow breached, if the City itself was laid to waste, so long as the Towers stood or the Qishir and their Court to whom they were tethered lived, then the City could rise again.
On the flip side of that, if a Watchtower were to fall? So, too, would the Dhaoine tethered to it. The Greywalkers designed each City in this manner and the magick that powers them to add extra strength to their Cities. It also meant that only one of their own could make additions to or tear down sections of each City safely without endangering everyone within it.
It was also said that the Watchtowers and the City they guarded were fed magick by the residents of each City and that with each dawn and each sunset, a sacrifice of power and energy was given to each Tower and then to the City itself. Though, this is merely speculation and not proven fact. However, many within the Worlds think it is credible given that with the genocide of every known Greywalker within the Worlds, their remaining 23 Cities are still alive, still thriving, though Shiran City is the only one that still glows the same golden hue it has since its creation.