Today, October 6, Paradox Interactive are celebrating Prison Architect turning 5 years old so they've done something a bit special to mark the occasion. Something of a special case as this is not an original Paradox title. It was originally made by Introversion Software but the rights were purchased by Paradox back in early 2019.
A game that speaks for itself, you're building and managing prisons. It's changed a lot over the years, through many small and big upgrades and now some paid DLC too.
Sometimes part of what people love gets lost as games are continually tweaked, so what Paradox has done here is quite sweet. They've uploaded the original release version from 2015 and then the last version released by Introversion directly in 2018. Both of which are available as an opt-in Beta on Steam. They also put up a little stats trailer:
If you wish to buy Prison Architect it's available on Humble Store and Steam, the classic downloads are not on GOG. It's free to play on Steam for the anniversary until October 11 with big discounts too.