Clash of Clans (CoC) is a mobile game for Android and iOS. It launched in August 2012, was the most lucrative iOS game of 2013. CoC was developed by SuperCell (SC), a Finnish game company founded in 2010. In April 2013, SC was generating 2.4 million dollars per day with only two games: CoC and Hay Day, a Farmville clone. In October 2013, a giant Japanese conglomerate bought half of SC's shares for 1.5 billion dollars.
I have played CoC actively from February to December 2013. I always played the game for free.
At the first glance, CoC is very much like Farmville: two resources, namely gold and elixir, are produced automatically over time. But resource production is perfectly done compared to Farmville: crops do not wither if they are not harvested on time. So it's always rewarding to come back and collect resources from the gold mines and collectors (mines, for short).
Most buildings can be upgraded by spending one of the two resources. Gold is used to upgrade defensive buildings such as cannons or walls. Elixir is used to produce troops to attack other players, and to upgrade troop-related buildings such as barracks or army camps. Mines can also be upgraded to produce more resources per hour, but also to store more resources until they are collected. For example, a level-2 mine takes 2h30 to fill up, which may encourage new players to visit the game more often. But players quickly realize that they are wasting resources during the night or a day at school. So players want to upgrade their mines to level 5 because then they take 10 hours to fill up. But upgrading a mine to level 5 requires Town Hall level 3, which itself requires a significant amount of gold, usually obtained after several hours of play. And that is how players get hooked.
Player versus player
After 3-4 days, the core mechanics change progressively. The amount of resources produced by mines becomes negligible compared to the cost of building upgrades. The player realizes that to keep her upgrades going, she must steal resources from other players, or from the solo campaign missions. The solo missions can only be completed once, so PvP is unavoidable. At this point, I suspect most players to take a decision. Some decide PvP is not for them, so they stop playing. Of those who continue, a very small minority decide that the game is all about competing against other players. These players are called trophy hunters, since winning a battle rewards the player with trophies. A leader-board shows the 100 players with most trophies. Most trophy hunters spend real money to instantly max their buildings and troops. They eventually become regular buyers, the 2% whales spending $200 per week.
The remaining 98% who decide to keep playing focus on stealing resources from other players. The most dedicated of them are called farmers; they train cheap armies and ignore trophies as long as they loot resources. Farmers are very practical and calculating: should they build an army costing 5-50k elixir in 15 minutes, and loot 100k-500k of resources from another player, or wait for their mines to produce 5k of each resource in these 15 minutes? Their behavior is obvious: 1) turtling is a waste of time, 2) they login when their army is trained, and 3) they logout when they have attacked and do something else (like playing another game) while their troops are training.
There are one elixir sink and two gold sinks. Elixir is sunk when the player trains expensive and powerful troops. Since farmers usually train fast and cheap troops, they often have more elixir than they can spend. The first gold sink is walls. Each of the 250 walls costs 8M gold to max out. The best farmers make 1M per hour, so we're talking about 2,000 hours of (sporadic) gameplay here. For someone playing 5 hours per weekday night and 20 hours during the weekend, walls take around 10 months. So walls are a humongous gold sink. The second gold sink lies in the match-making. When a player goes to attack, she is presented with another player's village. If there is too little loot, she can next for 200-1000 coins. Spending 50k gold in nexts to find a 200k-gold raid is very common. That second sink is very big too.
Pace of progression
To give an idea of the pace of progression, below are screenshots from my village from March to November 2013.