OK - actually fairly annoying on the hard levels if you just stare and try to work out the right sequence.

Unfortunately this is really easy with pencil and paper (or the digital equivalent): this made the hard puzzles a breeze - just circle pairs of numbers ensuring pairs are only used once until all numbers on board are contained within circles (erasing/adjusting circles as necessary), and then you can plan your moves easily... it would be nice to see the planning elements incorporated into the game: i.e. allow the player to circle pairs in a planning mode, and then in play mode, select the pairs (obviously you still need to do circle pairs in the right order in play mode - but at least you now can see the circles you intend to do in the gaps between the tiles... of course if you circle the pairs incorrectly in planning mode, then you'll be stuck - but hopefully you notice your plan is flawed and go back and change it).

While I think some of the hard puzzles are individually enjoyable problems, after ten of them, I was well and truly over the concept. It's just lacking something. Maybe use shapes or symbols or pictures of cats or tanks! (no actual need for numbers! in fact, you could have just used the coloured squares), and definitely choose a few upbeat tracks. Yes, I turned the music of pretty quickly, the single "calming" track hardly suited the game, and got annoying pretty quickly.