Thanks a lot! When I started researching, I wasn't able to find any commonly agreed standard rules, only countless rule variations (along with formalised rules for specific variants like Mexican Train). I also added this constraint where you are bound by a strict grid and you have to make your turns matter by playing doubles strategically, rather than allowing the player to turn whenever they like. So the rule set is a very customised one. I tried to communicate the essentials through the tutorial tips without overwhelming the player with info dumps but it's tricky to judge how much is just enough.
Glad you like the music, you're the first to mention it! Sound effects were a casualty of the time-limit, sadly, but I may go back and fill them in before I lose the momentum.