Once you start you can’t stop. That’s Clash of Clans. Right after the first launch of the game and in the middle of the tutorial sequence the attack of Goblins starts and you are sucked into to the game. There is no return back.
You will easily play Clash of Clans for many hours in a row without any breaks once you launch it for the first time. Game is heavily addictive and sticky factor is very high. You just keep on coming back over and over again. Let’s analyze why.
User interface is easy to get into, graphics are extremely stunning, sound FX and music live along the game experience and most of all you will not get bored easily as game has “depth”. Well-designed core loop, retention, monetization and social aspect mechanics support entertaining game experience. Let’s see how.
Well-designed core loop rewards player for being active and promises progress for each return session. The core loop of the Clash of Clans seems to consist of three different actions:
Image: Core loop of Clash of Clans 
Retention rate measures how effective are you at getting players to come back to your game. For example what percentage of the players who played your game in day one are still playing in day two. 
According to Ilkka Paananen from Supercell, the key indicators Supercell is tracking are 1, 7 and 30-day retention rates. Lasse Louhento from Supercell has revealed that for Clash of Clans the company breaks down the audience into three stages: newbies, mid-range and elite-groups. Each of the segments play the game different ways and have different retention rates. 
As you start to play the game for the first time you will notice the tutorial part just sucks you into the game and you keep on learning new things after another. And what is important, not all dependencies are available at the early phases of the game as those might confuse newbies too much. Improving the tutorial part has boosted the retention of the Clash of Clans. 
Newbies is the stage, players have to successfully complete to become valuable and engaged members of the community.
Almost all of the details in Clash of Clans are designed towards monetization (e.g. regular troops trainings and building your village). You always have a place to spend more money. Monetization method is based on the need of speeding-up your game progress. More you play, more time you spend to get achievements done. Or you could cut the paths and spend real money instead to fasten your progress in the game.
In the beginning of the game you have decent amount of free gems (in-game currency, which can be bought with real money). After few hours of playing you finally run out of free gems, because you have spent all your gems to gold and elixir (two soft-currencies, which can be cheaply bought with gems). At this point your psychology about gems is already formed; gold and elixir are cheap and you have purchased those with gems. Now you need to get more gems and you would get those by purchasing via the same “Shop” where you spend all your free gems. The threshold to purchase first gems with real money is low. Clash of Clans is optimized for the first purchase with real-money.
When you want to check how much real money you need to spend to get gems another psychological trick is used: price is presented in real money only when you are about to purchase more gems. Otherwise you keep on spending two soft-currencies: gold and elixir.
One aspect of monetization is a competitive level of game play; some people are willing to spend whole lot of money to be better and faster than others. One of the former top players of Clash of Clans, Jorge Yao, has reported to spend about 3000 USD into Clash of Clans in order to stay six months dominating the Top Players rank list. [4, 6]
These hard-core players (whales) are even treated with a special care in real life. Supercell has organized special events for paying players with free beer and snacks. 
Average Revenue Per User Estimates (ARPU) is estimated to be 4,60 USD and daily revenue estimates vary from 750†000 USD to 5,15 million USD. Monetization really seems to work. [7, 8, 10]
New York Times reported about a former top player called Jorge Yao, who spent over six months on top of the Clash of Clans ranking list. Jorge Yao gained virtual celebrity and became to social media star; he has gained over 30000 Facebook likes, almost 100000 Twitter followers and his Youtube interview has been watched over 400†000 times. How is this possible? How a single player of Clash of Clans could rise to social media star? 
Clash of Clans has several social aspects in it: First of all, player is regularly guided towards creating own Clan to play with friends and recruit new members to your Clan. Clan mates help each other by sending reinforcement troops. You may challenge your friends via Game Center with your Apple ID or using your Facebook account to connect. Your Game Center friends appear when you are logged into Game Center.
Secondly Global & your Clan message board is always visible and in use. You are able share your messages with other players around the world or only with your Clan mates.
Thirdly ranking lists (Top Clans, Top Players) include the possibility to view top player profiles, visit their villages (imagine you could visit any village and see how number one top player’s village looks like – awesome!) and view clan profiles. You have a possibility to search any Clans. Clans have two different types: invite only or anyone can join. All these encourage you to play together. Join your troops with your allies. And remember what Louhento mentioned about dividing the player audience into three groups. One group was called elite-groups. This explains the popularity of Jorge Yao also. He was part of the invite only elite-group called North44. Anyone could have viewed his village and see his profile on Top Players list for months. 
Also you have a possibility to play the game in a single player mode or multiplayer mode. If you choose to play the game in single mode, you still are encouraged to play together with others with all the previously listed features. You may anytime to switch from single player mode into multiplayer mode. This is encouraged with a visible Attack button. Logic is very smart: you may spend your real-money in a single player mode or multiplayer mode. And both modes are sticky and addictive!
Social aspect seems to work very well as Monthly Active Users (MAU) of Clash of Clans Facebook app has risen in 1,5 years from 1 million (in Q1/2013) to 10,2 million (in 06/2014) MAU. All Supercell's games (including Clash of Clans, Hay Day, and Boom Beach) combined to have 29,4 million daily active users on February 7, 2014.
Figures won’t lie, social aspect, game experience, core loop, retention and monetization all work better than ever. Try it yourself and you are hooked. [7, 9, 10]
This blog post is based on the Clash of Clans version 5.113.2 (current version is 6.56.1) 
 Gamasutra, Michail Katkoff’s blog, Mid-Core Success Part 1: Core Loops
 Gamesbrief.com, The business of games, Retention rate, churn and duration
 Pocketgamer.biz, 1, 7 and 30-day retention rates are key, says 'tablet first' Supercell
 Appfreak, Insider tips for mobile app marketing, Secrets behind the Clash of Clans monetization
 Arctic Startup, Clash Of Clans Meetup And The Supercell Success Formula
 New York Times, Master of his virtual domain
 Think Gaming, Clash of Clans
 Inside Mobile Apps, Supercell generating $1M a day
 Metricsmonk.com, Clash of Clans Facebook app usage metrics and charts
 Gameindustry International, Clash of Clans daily revenue at $5.15 million - Hacker
 Clash of Clans Wiki, Version history