It’s rare to find a game that’s a financial success before it’s even launched, but Prison Architect is a great example of the early access sales model.

The prison simulator officially releases for PC on October 6, and developer Introversion Software has already earned $19 million from it, reports GamesIndustry. Prison Architect first came out in a paid alpha form way back in September 2012. Since then, it’s sold over 1.25 million copies during its ongoing development, via Introversion’s own website and, more recently, the Steam digital store.

Prison Architect was one of the early pioneers of the paid alpha model that also served the block-building phenomenon Minecraft so well. Recently, we’ve become more used to the idea of buying content that’s not finished, thanks to crowdfunding sites like Kickstarter and Valve’s Early Access portal on Steam.

Despite Introversion’s success with Prison Architect, the indie developer remains cautious. It nearly closed back in 2010, four years after releasing real-time strategy title Defcon. “After Defcon’s launch we were in really rude health, really solid health, and within four years we were almost gone,” said Introversion co-founder Chris Delay. “You can blow your money very quickly in video games.

“We think that Prison Architect could be the biggest hit of the next 15 years at Introversion, so we’ve been very prudent with what’s happening with the resources that it’s generating, because we know that we can never run out of money again now. Not because we’ve made so much, but because it would be criminally negligent if we ever had to go back cap in hand.”