From the Design Wiki:
Urbis - A World of Cities is a fantasy setting developed for the Dungeons & Dragons RPG by Wizards of the Coast.
So, what is Urbis all about?
Urbis is Urban Fantasy
As the name implies, cities are central to Urbis. Gone are the quaint market towns of other fantasy worlds - these belong to an earlier, feudal age. Instead, the world is divided into a multitude of city-states with populations ranging into hundreds of thousands, or even millions of people! While there are plenty of wilderness and frontiers left in the world, these are not the main focus of this setting. There are plenty of things to do in the cities, from hunting monsters in the sewers, getting caught up in struggles between crime families, fighting for social justice for the downtrodden masses, to high-stakes games of diplomacy and intrigues between the rich and powerful of the cities.
Urbis is a Magical Industrial World
While most RPG fantasy worlds draw their inspiration from the Feudal Age or Renaissance of our world - or even earlier periods - Urbis is heavily inspired by the time of the European Industrial Revolution - the time when cities came into their own. On the one hand, it was a time of wonder - of scientific and technological prowess, and it seemed that there was nothing that humanity couldn't do. On the other hand, it also saw the rise of the urban poor - of masses of desperate people huddled together in ramshackle apartments who had not only work themselves to near-death, but also had to send their underage children to the same factories to keep their families from starvation.
The Nexus Towers, the iconic image of Urbis, represent both of these extremes. On the one hand, they promise near-limitless magical energies to those who control them, and thus make the vast cities with all their splendor and magics possible. On the other hand, they draw upon the very life of all those who live nearby, thus also becoming the Satanic Mills that many people imagined the early factories in our world to be.
Urbis is Realistic High Fantasy
At first glance, this seems to be a contradiction in terms. After all, "realism" and "fantasy" do not often come together. What this means in the context of Urbis is that for all the magical trappings of the setting there is an inherent logic to it. Fireball and other spells have radically altered the face of the battlefield. No longer is a battle fought with tight infantry formations that just ask to be killed by area-effect spells. Instead small groups of warriors try to sneak into enemy territory so that they can take out key installations or people before the other side notices. Raise dead has a real effect on the social structure of the setting - the rich and powerful are unlikely to die permanently of anything except old age. And all those monsters that look like they couldn't have possibly evolved in an Earth-like environment... well, they didn't. Most of them were brought to this world from other planets.
To sum it up, nothing in here exists in isolation from the rest of the setting. Everything affects the rest of society in some big or small way, or at least has the potential to do so - if the player characters bring it to light.
Urbis needs Heroes
While the cities seem calm and civilized on the surface, below it are intrigue, social unrest, and hidden wars that could turn into open bloodshed at any moment. As the cities grow ever larger and the differences between rich and poor more pronounced, some sort of radical transformation or even outright revolution becomes more and more likely.
Something has got to give. The only question is: In what direction will you push it?