The magic in SiT isn’t divine or arcane, there’s no such division.
The site on which Legacy is built is infused with magical energy and that… will suffice to explain magic.
Perhaps it was the First Dwellers who instilled it. After all, they were big fans of bloodletting sacrifices (and to put it bluntly: a fucked up bunch) and everybody knows there is power in blood.
Another explanation comes from the Urbi cultists, fanatic believers that worship the Spirit of the City, a goddess slain long ago on the very site of the City and now “embodied” in it. For them, She IS the City.
Maybe magical power was always there, like a random geographical feature, and those who could benefit from it were drawn (or lured?) to the place.
Or then again, it could be all three together or none of it…
Whatever the truth, the result is magic manifesting itself way more pervasively in Legacy than in most places in the World (1). In some spots of the city that are called Nexuses (plural of nexus, I just learned that), it is even more so. Those that can harness them can gain magical powers, some subtle (small buffs), others not so subtle (fireballs and such).
Also, the City “heals” itself. Nobody has anything to do with repairing the walls, roads or bridges, they just mends if needs be. Very cost-efficient!
- Not that it matters, all will take place in the one city.