Haha I Google translated the whole thing (the website and the app) on my phone. From what I could understand, you’d download an app that apparently translates to “that boyfriend” or “AnaKare” from the AppStore or Google play store (this is a link to the AppStore version of the app cos I’m on my iPad: https://apps.apple.com/us/app/%E3%81%82%E3%81%AA%E3%82%AB%E3%83%AC/id666784253 ). When I looked up the app it looked like it had lots of other stories on it so you’d probably have to hunt for caste haven specifically.
The game looks to play like a visual novel, so I imagine you’d play as one of the main boys. It has exclusive artwork from the mangaka in the game as well so it’s not some third party thing.
However the entire thing is in Japanese so if you can’t read or understand it you’re either gonna have to translate each page somehow or just not play at all.
Caste heaven has a phone game! here's the site guys. Anyone here know how it works?
http://caste-heaven-game.com/?a=tw