The cosmic entities that forged the world got a little more than they bargained for. Even the most powerful of them were but limited aspects of the underlying magic of the cosmos. The world they created, and the races they blew life into are not so limited. In particular, the sapient races of this planet carry within them a more prefect blend of these aspects than the gods have, though they have far less power. As a result, after years of bowing to the gods' ever more cumbersome and capricious rule, they rebelled, and wielding an aspect of destruction alien to the ruling deities, they killed them off. Humanity was left with the ruins.
And onto the question:
So we're now left in a world with massive edifices the gods built to house themselves. Lets assume they're made well enough to stay standing for at least a few centuries, aren't so big they just crush through the planet's crust, but are made of materials that humans can work.
Humanity has a vested interest in building its ruling cities in these places, as the ambient magic proves useful for the ruling mages. Further, they don't want to destroy the ruins completely for fear of the same fading.
So, what do these cities look like, how do they grow? Lets suppose that most god-dwelling contain a few spires, some sweeping walls, and sit atop a nice plateau. Most of the internal rooms will be far too big for a human to use as a single space unless they are using it as some sort of large open square/marketplace/field/park.
How do you build inside a building?
The main inhabitants of the upper cities are in a roughly Early Modern technology level. Largely pre-gunpowder and steam (though a part of this is societal taboo, so other areas may be better along). However the nations they rule are non-magical and are in more of an early industrial stage, so they may have some benefits of this (Again though, taboo segregates)
Magic is plentiful, if somewhat dangerous. It is shipped in bulk as a magical mist/condensate from certain areas where large numbers of gods died in the rebellion of mankind. When properly channeled it could be used to achieve large projects such as levitating large objects/bulk material and stengthening or enchanting building supplies.