Warhammer: Vermintide 2 launched on PC just last week, but it has already surpassed 500,000 copies sold. Developer Fatshark debuted the cooperative melee action adventure March 8 for $30, and fans and critics have praised it for building on the original’s gameplay with refinements and more substance. And now that praise has turned into brisk sales.

Fatshark chief executive officer Martin Wahlund explained that the rate of sales has exceeded the studio’s expectations.

“We are proud and happy that gamers worldwide have taken Warhammer: Vermintide 2 to their hearts,” Fatshark chief executive officer Martin Wahlund said in a statement. “500,000 games sold was a milestone that we had, but not less than a week after release.”

Warhammer: Vermintide 2 expands on the ideas of the original, which plays a lot like a melee-focused version of Valve’s Left 4 Dead games. The original Vermintide, which debuted in October 2015, also sold well — but Fatshark notes that game took six months to reach 500,000 copies sold.

In Vermintide 2, players take on familiar roles of elves, dwarves, and magic users. And because the world is ending, you must work together with a team of others to wipe out the Chaos and the Skaven horde — as you might expect from the game’s name, those opponents often come in the form of giant bipedal rats.

While the action works like Left 4 Dead, Fatshark has attached a strong progression system to that structure. You can choose from one of five heroes, but each of those have multiple career paths that you can spec toward. This should help keep the game fresh as you play through it over and over with friends or with strangers.