Nice idea for a strategy game, well executed and appropriate graphics and sound effects. In practice there are only a limited number of routes to try so it can become a bit trial-and-error.
The last level is a bit cheeky, it would have been better to have introduced earlier on in the game the concept that the "one-way" squares only prevent you moving backwards ONTO them, but you can LEAVE them backwards. Discovering that mechanic only at the end of the game is a bit harsh! I never care about medals so this isn't a big deal for me, but I notice some other people are complaining about them not working for me. Just in case this helps you with debugging, I can confirm all of the medals work for me except for the final "complete all 24 levels" medal - despite me reaching the "thanks for playing" screen and beating all the monsters there.
Probably the "thanks for playing" screen should link back to the level select menu just in case someone wants to try again? You might add a bit of replayability by giving a star rating for some of the levels for how many hearts remaining, and recalibrate things slightly so that all levels can be beaten with at least 3 hearts remaining on a flawless attempt but you can get through with 1 heart remaining on a "half-decent" attempt. Obviously this means allowing players to get on to the next level with less-than-perfect strategies, but that might be a way of mitigating the high level of difficulty of the final levels (at least it seems so given the number of people in the comments who are asking how to do them, although I don't think they're that hard once you work out there are only a limited number of routes through that need trying!).