Interesting Idea
1) Some people complain about AI being too strong. Well I beat it in my first attempt and I do not play chess that much. Last time I played chess was about a year ago.
2) Changing Color of all possible moves when you click a piece would be good for beginning players (I'd love it, because I always need some time to figure out where are the available squares for the horse).
3) The rules, reading your tutorial I heard the en passant for the first time. Ok, nice. About the casting, I heard that you can only do this only if no pieces were attacked, but there are variations probably, the time I heard it was when two of my friends were having a huge argument whether this rule is true or not.
4) Animations were nice, I liked them.
5) But in the end I believe that animations were the only thing that distinguishes this from other games and sadly such detail cannot easily become core concept which makes the whole thing really unique or great. Adding multiplayer would be easy, maybe useless, but easy and it would not burden singleplayer experience. Making more games available with animated characters or maybe including some "minefields" campaign.
In the end: 8/10 A good game, with a little twist. But the twist is just too small to make it perfect.