"Our core players are very much women, aged 25-plus," Mr Knutsson says.

"And the people who do pay, spend about $25 [£19.20] a month. That's not a huge number per player, but our revenue comes from a huge number of players."

Bug fears

The 2,000th level is intended to be a tempting treat that attracts back those who had set the game aside.

King says hundreds of thousands of players had progressed up to the 2,000th level, but it is making it available to other players too

It features a story inspired by the fears computer systems would fall foul of a "millennium bug" at the end of 1999, and

That's just as well, as King has struggled to replicate its success in genres beyond the match-puzzler and some are concerned about how long their appeal can endure.

"King reported it had 409 million monthly active users at the end of June 2016," notes Jack Kent from the research firm IHS Markit.

"That's its lowest level since 2013 and a drop from a peak of 550 million users in early 2015.

"It still has a huge audience to monetise… but it needs new intellectual property or a revamp of its existing titles to achieve significant growth."

Sebastian Knutsson co-founded King in 2003

Mr Knutsson counters that King's titles are prone to "seasonality" and typically do better in the "winter dark months".

But he acknowledges that it's been "hard or even impossible" to replicate the success of the match puzzle titles.

"We have a lot of games in development - but the bar of launching a game is increasing all the time," he says.

"We start with prototypes, we'd expect half of those never to get to production with a bigger team.

"And when you do go to production, the game may not be as good as you think or player reception is not what we expect, so at least half are culled again.

"So, the majority of the game ideas that we start never make it to market."

Hero - a fantasy-themed role playing game - and Paradise Bay - a tropical island-set simulator - prove the developer is trying to stretch itself, but neither has made much impact to date.

Paradise Bay and Hero mark attempts by King to launch different types of games

Mr Knutsson confirms King is also working on a shooter, but declines to reveal more.

Incoming ads

So, for now, King's fortunes depend on the Saga series, and a forthcoming feature could prove as lucrative as it is divisive: the introduction of adverts.

"The vast majority [of players] never pay anything within our games," Mr Knutsson says.

The developer of Mobile Strike raised awareness for the game by running video ads for it within other titles 

"We could offer them more features if we could use advertising to create funds."

King's plan is to offer in-game bonuses if players agree to pause their play to watch short videos - a tactic already employed within Angry Birds and elsewhere.

Mr Knutsson says it will be a "win-win" for all involved. But the risk is that the interruptions will undermine the addictive spell the games cast over players, weakening their hold.

From the outside, it might look like Activision is trying to exploit King's assets before interest in them wanes.

But Mr Knutsson denies being pressured into making the move.

"We did ads within our Facebook web games three years ago," he recalls.

"When we moved to mobile we didn't have enough time to focus on that business model.

"I think it's a signal of the maturity of the mobile industry and our games that now is the time to start to explore that area again."

Social gaming

Pokemon Go proves a new type of game can still capture the public's attention.

Pokeman Go superimposes its monters on top of real-world views

In July, the monster-hunting app overtook Candy Crush Saga's record for having the highest number of US players active on the same day.

There are doubts about whether Pokemon Go will have the same staying power, but King isn't ignoring the competition.

"To me, it's not the augmented reality that makes it interesting, it's more the physical aspect of walking around and the social aspects it has created, where you interact with strangers that you meet about town," Mr Knutsson says.

"We don't look at trying to be the first on new technology - whether that's virtual reality or augmented reality.

"But anything that has mass market appeal is quite interesting to us as that's our audience, essentially.

"Having games with a physical connection to the real world is definitely something we will explore at some point."

He adds that King will also keep a close eye on Nintendo's launch of its first Mario title smartphone later this year.

"It's going to be a threat for stealing time from our players - it's going to be interesting to see what audience they appeal to."

Indie challenge

What Mario, Pokemon and, indeed, the Saga games all have in common is the power of strong brands.

A few years back - when King and other indies rose to prominence - it seemed like app stores might act as a kind of primordial soup, allowing new ideas from small studios to prosper, in contrast to the console sector where big publishers and their sequels dominate.

King hopes the 2,000th level will boost use of Candy Crush Saga ahead of its roll-out of adverts

But Mr Knutsson suggests it is unlikely many of today's indies will be able to replicate King's success.

"Very few new entrants have made the top 10 lists in the past 24 months," he comments.

"The demands on your skills at marketing a game, the demands of working with data, the investment you have to make in a team - these are continuously going up.

"So, it's harder and harder for a small indie team to achieve this than was possible.

"Today you need a certain experience level and size to make something that cracks into the top 10."