Despite Prison Architect being 'finished' for quite a few months now the developers have returned with yet another update. Like with Update 11 there are no massive new and groundbreaking features to be found here, but rather a whole bunch of important fixes and improvements, mostly focused around the staff and the 'super-secret' 3D mode.
As always, the developers have released an extremely lengthy video detailing every single change and tweak they've done. If you're interested, and you have 30 minutes to burn, I would recommend you give the video below a look as its surprisingly entertaining:
As far as specifics are concerned, you can find the full patch notes over at the official website. Just do keep in mind that Update 12 is currently in beta, so you will only be able to access it by going to Prison Architect in your games list on Steam, clicking Properties and then Betas, and then selecting the newly added 'beta' option. Once you've done all of that the update will start automatically, but if you end up having trouble just restart Steam - that usually fixes it. Have fun, and don't forget to report any bugs!
