Fortunately, no city is an island (even when they're on an island). The focus on multiplayer plays a central role here, and it's difficult for a single city survive on its own--if you'd like to see any variation, that is.
A titan of industry
Here's an example of what I mean: Let's say we'd like to create an industrial powerhouse. Factories need employees, so we'll need plenty of residential zones with low-income citizens looking for a humble nine-to-five gig. But our factories will also need places to sell their wares--that means exporting goods to the new trade depots and ensuring there are plenty of commercial zones available. Factories also cause quite a bit of ground and air pollution that drives down land values (and make your citizens unhealthy), keeping wealthier business and residents at bay. We'll mitigate that by ensuring there are plenty of schools and libraries to educate our populace: educated citizens can become skilled managers, resulting in high-tech factories that emit less pollution.
We're almost done. Factories are fire hazards, so we'll need comprehensive fire coverage. And we can't all be mid-level managers, so we'll need a stable supply of low-income residents. Even with high levels of education, low-income areas generate crime, which means having comprehensive police coverage.
All of these buildings require power, water, and--most importantly--lots of space, if they're going to grow. A thriving city also needs a sensible road layout: larger roads are more expensive, but can ease congestion in larger cities--our police and fire departments can't do their jobs if they're stuck in traffic.
The prudent industrial city planner needs to cram plenty of city services (don't forget public transportation for your low-income citizens!) into the same space as residential neighborhoods, commercial plazas, and industrial corridors. Add a few power plants and water towers to keep everything humming along, and our city will quickly be packed to the gills. And all of this only accounts for the bare essentials. There's gold in them thar hills: we can strip mine parts of your city for resources like coal and oil, and use them to fuel power plants, trade on the global market for cash, or refine into advanced materials for use at advanced factories. Wealth is at our fingertips, but only if we can find enough space to fit everything in.
The air isn't great, but we've got plenty of jobs!
EA's SimCity present a "convenient" solution: team up with someone else. Every region offers a number of cities to build in, and cities in a region form a cohesive unit. It's still called SimCity, but SimOrganism is closer to the mark: each city serves as a kind of organ, and successful cities (and thus, regions) will share resources intelligently.
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