[In the first of a two-part series, design veteran Tadhg Kelly draws from his What Games Are blog to explain the rise to power of Zynga's massively successful social game CityVille. (UPDATE: Part 2 now posted.)]

Check out the latest Appdata graph for Zynga's Cityville - that's right, it now has nearly 70 million monthly active users, and the Facebook game only launched in early December.

I know what many of you are thinking: How does Zynga keep doing this?

At the Getting Social event at BAFTA (In London) a few days ago, this was the question that everyone was asking. While TV companies in the UK have dipped their toes into social games, such as Corrie Nation, they have had pretty miserable success rates.

And yet here comes CityVille, another Zynga game that looks quite a lot like other developers' games, they waltz in, do their thing, and boom! 12 million users in a week, and multiples of that shortly afterwards.

It's not just Zynga. Although clearly the most successful, the other top developers also manage to up-end the natural order of things as most media people understand it (which is to say, brands).

At the same time that Ubisoft have managed to scrape together 1.2 million users for their CSI: Crime City title, another game more generically named Crime City (no relation) has acquired 6.4m users, with no brand at all.

I decided to write an article about how games like CityVille manage to be successful. This article, which will come in two parts, goes into the specific features and explains what they do, why they work, and what I think they could be doing better. Hopefully it will give you some idea not just of what social games are doing right, but also why players might play games of this type.

What is CityVille?

CityVille is the latest in a series of city-building games on Facebook, and was released about two weeks ago. It is the latest in Zynga's range of light sim-style games (FrontierVille, Café World and of course FarmVille being the main examples), and very much in the same vein as other titles like Social City, Millionaire City, City of Wonder, or My Empire.

It has taken Zynga quite a while to get in on the city-building game theme, but they have taken the time to build out their own game with their own mechanics rather than the more direct copying that they and many developers practised in the very early days.

Starting with a couple of streets and buildings, the game guides you through various tasks that you can perform (sowing crops, building bakeries, laying road, etc.) in bite-sized chunks. Rather than throw all of this detail at you at once, the game intersperses it with challenges, things to pick up, collect, friends to visit and so on.

It is click-heavy, meaning that to build a building you don't just place it and let it be built. Each click builds a phase of the building, and there may be three or four stages before a building is actually finished. As buildings generate revenue, need supplies, or crops are grown, the game does not automate those actions. Instead you manually collect coins, deliver boxes, pick up prizes, click to plant crops, click to harvest crops, and other actions.

This activity all proceeds a-pace until you run into timers. You can only collect coins from a building every X minutes, for example, which encourages you to check into the game one or more times a day. And, globally, the game monitors all your actions with an energy statistic that either regenerates over time, or you can buy more of with the game's cash.

As you do these activities more and more, you earn experience points (or XP), which increase your level. Levels unlock more buildings and rewards, which in turn let you make more stuff, increasing your maximum energy. And for a bonus effect, your energy recharges every time that you gain a new level.

You can also visit other player's cities. This has the effect of immediately giving you energy awards and boosts, as well as offering activities that you can do. You can harvest other players' crops for them, which generates mutual awards, such as XP, reputation points and game cash. You can apply to other players to let you set up franchises of your businesses in their cities. You can also send them gifts which cost you nothing (such as free energy).

The game continues to guide you with tasks. A task is usually quite simple, involving three or four steps. Steps might be include visit three friends, build a bakery, collect ten strawberries or that kind of thing. You may have several tasks on the go at the same time, as the game monitors whether you're completing steps or not, but not all tasks are immediately available. Instead, the game chains them along, with completion of some tasks opening up other ones.

Task completion generates congratulation windows, rewards, and also the opportunity to share your achievement on Facebook.

Lastly, you are largely free to lay out your city as you choose. Unlike many classic sim-strategy games (or Restaurant City, arguably the grand-daddy of these types of games), optimal layout doesn't really matter. You don't need to maintain equitable balances of components in certain areas, efficient road networks or anything like that. All of the people wandering around, as well as the plants and trees are purely decorative. This means players are free to create whatever layout they desire, and many do.

And that's the basic game design. Most successful social games are much the same, but with variations of theme. So why does this work so well for Zynga, if many of the games are the same?