Interesting twist on an old concept, to result in a fairly entertaining game. The fact that I actually had to think (a bit) while playing this is a very definite plus :-)
However, there were a couple of things which I think might be improved:
At the moment, whenever two groups of three are created simultaneously (e.g. in a T shape), only one of the two is removed. Why not both?
A slightly bigger quibble is the way that the gravity in the game works. I can understand the idea of keeping the original tetris gravity, but I don't like the fact that when a horizontal group of three that has other blocks on it is removed the remaining blocks blocks just stay in the air. In tetris, whenever a line disappears, all the blocks on top fall down one square. Why not do the same here.
Overall not a bad game though; I would give it 3.5/5
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