Out of the blue, Prison Architect has added cooperative multiplayer.
It's far from finished and requires opting into alpha/beta updates on Steam if you want to try it, but once enabled you can happily go about building a top-notch prison with your friends (up to seven).
There's plenty that doesn't work in multiplayer because the game was never meant to have it, but Introversion has plenty of ideas about how to flesh it out and improve it so long as you want it. If everyone who tries it hates it then Introversion probably won't bother.