There are many reasons it's very difficult to create a commercially successful F2P mobile game.
Fundamentally, however, it boils down to the issue of how do you appeal to the widest global audience with an experience that offers the deepest level of meta/gameplay?
The problem, of course, is these two elements are inversely related.
The deeper you make your game, the fewer people it will appeal to.
Let's consider two extreme examples of very successful games to understand the issue.
King's Candy Crush Saga is a match-3 game that anyone can play.
While there is some depth to the gameplay - notably its increasing difficulty - there's no real meta-game other than seeing what level of the game your friends are on - social coercion.
Cards immediately make us feel smarter.
In this way, the game appeals to everyone and once you understand how to match three symbols together that's all you need.
In terms of our appeal versus meta-game construct, Candy Crush Saga has very wide appeal but offers a relatively shallow experience.
A different approach
Conversely, Machine Zone's Mobile Strike is a game with little understandable gameplay.
When you start playing, big onscreen arrows point where you should click. This continues until you give up playing, somewhat confused, or you knuckle down and join an alliance.
That's when the incredibly deep meta-game - which sees players paying hundreds of dollars every month to keep up - kicks in.
Mobile Strike is a very niche in appeal, because it offers an incredibly deep experience.
Marked with a crown
In this context, the genius of Finnish developer Supercell is that all its games sit inbetween these extremes, in a sweetspot that appeals to a wide audience, while also offering surprisingly deep monetisation.
Wide and deep, latest release Clash Royale is the prime example.
The first thing to note when you start a Clash Royale session is how quickly the game loads.
All Supercell games load within 20 seconds, typically around 10 seconds.
There's no waiting around to log into a server or for additional data to download. That's fine for niche RPGs that hope to monetise with a deep meta-game but not when you're aiming for 100 million downloads.
Indeed, combined with the headline figure of '3 minutes per battle', a player can tell themselves any Clash Royale session will take a maximum of five minutes, even 20 minutes later, when they're onto their six or seventh battle.
The next thing to consider is the flow of the user interface.
The items you gain are worthless, but psychologically you feel good about the game.
While there are a lot of icons onscreen, a limited set of clear options are highlighted.
Typically, there will be an animated Free Chest Open option to tap on, which is a basic retention reward for starting the app.
In a clever manner, the items you gain are pretty much worthless, but psychologically you immediately feel good about the game.
It's called reciprocity.
Even smarter, if - like me - you vigorously tap to open the various items in the middle of the screen, you'll likely accidentally tap on the big yellow Battle button once all the Free Chest items are unlocked, taking you into a battle.
But that's okay, as a battle is only three minutes long.
In terms of battles, Supercell's choice of cards for the core gameplay mechanic is also clever because cards immediately make us feel smarter - a halo effect from Poker perhaps - as well as seeming to offer more tactical opportunities than the game actually represents.
That's not to say that Clash Royale doesn't have tactics. Only that the game's matching system means that for the vast majority of players outside the top 500, the win-loss ratio quickly becomes like tossing a coin: heads you win, tails you lose.
For the vast majority of players, the win-loss ratio quickly becomes like tossing a coin.
Significantly, the game doesn't display your win-loss rate either.
You can go into the stats and find out how many games you've won, and your trophy total provides an abstract ranking so you can compare yourself to other players.
But there's no overt measure of your success in terms of the number of battles you've actually played.
Everyone's a winner
More fundamentally, there is an argument that everyone playing Clash Royale (apart from the worst player in the world), will have a good experience, because the second worst player in the world will beat the worst player more often than they will lose to them.
Personally, I'd take this argument one step further and claim that for most players, it's not too difficult to win in Clash Royale by randomly selecting units (i.e. as they become available) and dropping them onto the board.
In this respect then, Clash Royale is reminiscent of Candy Crush Saga. Almost anyone can play it and rack up some victories, open some chests and win some items.
In that respect, it's a game that appeals to losers (and that's everyone apart from the best 500 players in the world) because it makes them winners - at least for half of the time.
Okay, so it's not based around colourful candies, but even compared to Clash of Clans', I think Clash Royale has a broader appeal.
After all, it's a card game, and everyone's played card games.
Bait and switch
Supercell has cracked the wide appeal part of the formula for success, then, but Clash Royale's real genius is in the way it hides deep and 'aggressive' monetisation.
(BTW: I use the term 'aggressive' not in a pejorative way. 'Aggressive' doesn't mean good or bad. It's just describing how the game's monetisation operates. After all, this is a game in which you can spend over $25,000.)
Clash Royale's real genius is in the way it hides deep and aggressive monetisation.
The most obvious example of this is what happens when you've won a battle.
'Yeh, I've won a battle, so where's my reward?' goes your brain.
Your reward is a chest of items to be unlocked. But what's this? The chest is locked and you have to wait a minimum of three hours for it to unlock. Or you could spend some gems (hard currency you can buy) to get your reward immediately. Hmmm...
As with most F2P games, Clash Royale gives players some gems when they first start playing to encourage them to open chests immediately.
It doesn't push players into doing this, nor is it generous in terms of how many gems it gives you.
From that point of view, the decision is very much left down to the player. Only that, from a psychological situation, you'll likely want your reward more than you care for your gems.
And, for me, this is the real genius of Clash Royale.
One of its core monetisation loops (the other is the gacha mechanics of buying chest and getting high level cards, but that's another article), is embedded within the core reward loop, but in such a counter-intuitive way that delayed anticipation of the reward actually enhances, not detracts, from monetisation.
No other mobile game developer would be able to pull off such a move, because if they attempted it, they would be meet by a wave of complaints.
But given that everyone - apart from the worst player in the world at Clash Royale - is having a good time, most of the time, we seem to have missed this trick.