In the first part of a new Gamasutra feature two-parter, design veteran Tadhg Kelly examines some of the factors that have turned Zynga's CityVille into the company's latest overnight success, attracting over 70 million players in less than a month. One of the keys to this stratospheric player acquisition, Kelly argues, is the visibility and player base granted to the new game by cross-promotion through other successful Zynga games. "If you are sitting on an ageing 53 million monthly active users in FarmVille, as Zynga are, why not show them something else that they can play?" Kelly asks. "Each user that does this becomes a more invested customer, more likely not only to play your next game, but to still keep playing and maintaining their existing game." Once Zynga has attracted initial interest from its existing player base, Kelly says, the heavily click-based gameplay keeps those players engaged. 'So much clicking is oddly compelling," Kelly says. "The player doesn’t actually have to click to do everything ... but there’s a nice feeling that comes from such activity. It’s interactive, and that in turn makes the game mildly immersive by making the player feel like they are doing something, even if that something is essentially just sweeping up." To keep players clicking, Kelly shows how the self-imposed limit of in-game timers and the slow drip of rewards and unlockable content make players want to see what's next. But the game's real addictive potential, Kelly says, comes from a never ending series of overlapping tasks that can never be fully completed. "No matter how quickly you play or how much money you spend, there is always something to do, some gate to unlock, some task tree to complete, some daily bonus to claim, some new set to gather, some crop to harvest or some level to attain," Kelly points out. "The loops that the game creates in your mind cannot be closed until you come back later. In the mean time, have a cake!" The complete feature goes into much more detail about how the games mechanics work, and the psychological concepts Zynga uses to make those mechanics so compelling to so many.