50 very quick explanation: create a dungeon, control minions and kill stuff.
You can buy the sprite version for $15, ASCII art version is available at github
https://github.com/miki151/keeperrl
If you go to the download page you also see this quote
thus if you like animals more then developers but also want the game, that seems like a good alternative to get the sprite version
Also, if you can program in c++ and want to peek around (or maybe add features yourself) to the source code of the sprite version, this sentence should catch your eyes
thus I strongly suspect you can even get the source code of that version. (Also, its a GNU license on the ASCII version so yea..you will get the source code and you can do whatever with it unless there is something I am overlooking )