No other mobile game has done this so quickly in the age of Clash of Clans, Game of War: Fire Age, and Candy Crush Saga. A brand new game, Supercell’s Clash Royale fortress-battle title, has become No. 1 in the U.S. — not only on the top downloads chart but also on the top-grossing list on the iOS App Store.

That is no small accomplishment in the $34.8 billion mobile-game industry, considering that Clash of Clans alone generated well over $1 billion last year in revenues. That means that Supercell’s Clash of Clans is generating multiple millions of dollars a day in revenues, and now Clash Royale is beating that.

A lot of games hit No. 1 in top downloads as people discover them. But it’s rare, as mentioned, for anyone to break into the top-grossing list that has been dominated by Supercell, Machine Zone, and King. Then again, it is no surprise that Supercell, which has lots of fans and multiple hits, is the one to dethrone its own No. 1 title on the top-grossing list. Clash Royale is also No. 1 on Google Play downloads in the U.S., but it isn’t showing high on the top-grossing list just yet.

This is Supercell’s fourth hit after Clash of Clans, Boom Beach, and Hay Day.

Ilkka Paananen, the CEO of Supercell in Helsinki, Finland, said on his Facebook page recently that he was happy to release Clash Royale, particularly after the company had killed so many other games. Supercell rigorously tests its games in certain markets before releasing them. One of them, Smash Land, was pretty far along in development when the company decided to kill it in mid-2015. That kind of religious focus on quality has earned Supercell some loyal followers, and it tells you a lot about why Clash Royale is so important.

When telecommunications company SoftBank purchased a controlling stake in Supercell in June 2015, Supercell was valued at an astonishing $5.5 billion. Its success has spawned a lot of other mobile-game startups in Helsinki, and everybody is chasing the company’s success. But Clash Royale shows that Supercell still has huge advantages even as it remains a relatively small game studio.

It’s no wonder that Apple gave the new game a huge feature position in the App Store when it launched on Wednesday. Clash Royale is a blend of Clash of Clans, real-time strategy, and multiplayer online battle arena (MOBA) games such as League of Legends. But the blend itself makes for something uniquely addictive, as I found in my own play session.

Clash Royale goes further than Clash of Clans — which has asynchronous battles, where one online player attacks and an offline player defends — in switching to synchronous, real-time combat. For this to function smoothly on mobile requires some huge infrastructure. I haven’t seen a hiccup yet in my battles against real players. It has a slight loading time at the beginning or end of a match.

You have a slate of characters that you can drop onto the field, and they start moving slowly toward the enemy’s defenses, which consist of two forts and a main castle. It has two lanes for combat, in contrast to five for typical MOBA games. The options for playable cards show up in the bottom of the screen. You get a choice of four. A “mana” meter runs along the bottom. If you have enough mana to play a card, it shows up in colors. If you don’t have enough, a timer that runs in real time shows you when it will be ready. When you are ready to play a card, you just press down on it and move it to the right part of the screen where you want it to appear. That’s all you are required to do. That makes the gameplay repetitive and simple. You just have to think fast.

Image Credit: Supercell

It has some similarity to Hearthstone: Heroes of Warcraft, the hugely successful collectible card game from Blizzard. You play cards and choose from among different ones in a deck, and you have to think offline about what kind of cards you want to get (and in this case, upgrade). But Clash Royale is much simpler.

Clash Royale has just a few dozen cards at the moment, but most of them are easily recognizable as characters from Clash of Clans. If you drop down a giant, it will lumber toward the enemy’s fort at a slow pace. Once it gets there, it can do a lot of pounding. Your fort can fire back, but you have to drop some kind of defense, like a dragon, to counter the giant, which can squish archers and goblins quite easily. Each character has advantages and disadvantages when facing other units on the field of battle. It’s a “rock-paper-scissors” kind of battle where you have to decide within seconds what to do.

The battles can end within three minutes. If you run out of time, the side that has destroyed more forts wins. And if you take out the king’s fort, you win. If the game is tied, you go into a sudden death for one minute. I’ve managed to win or lose a number of battles with just a few seconds left, and that’s what makes Clash Royale so exciting.

After each battle, you get a reward. A small wooden chest usually yields poor loot. But silver or gold chests release better stuff. It takes 15 seconds to open a wood chest. But it takes three hours to open a silver one and eight hours for a gold chest. You get better loot, but if you want to speed up the opening time, you have to spend gems, and those gems can either be earned in combat or purchased with real money. Those gems are the reason that Clash Royale has already hit No. 1 on the top-grossing list on the U.S. App Store.

Joony Koo, the director at Big Pot Games in South Korea, said he noticed these things about Clash Royale. The play time fits well with mobile games, at 150 seconds to 210 seconds maximum. You can pick up and play, and you can easily drop it as well since losing doesn’t really impact the play. It has complexity in both defense and offense, depending on which card deck you are using. It mixes multiple genres, including card battle, defense, role-playing game, and strategy.

He noted the game reuses all of the characters and their characteristics from Clash of Clans. It is easy to learn and hard to master. It has tons of meaningful upgrades, swift and seamless connections for player-versus-player. You can focus on a simple guild system around the metagame, and there are daily quests where you’re collecting. That helps retain users and makes for limited grinding. On top of that, it is good for esports as broadcasting of Clash Royale has already started on multichannel networks.

Now the question is how long it will stay at No. 1. But lord knows that Supercell has a huge ad budget, and it hasn’t even begun to advertise the game for real yet.