I like that the "towers" are flexible. One thing I thought was odd, though, is you can't seem to upgrade a unit unless all 3 units have been purchased.
Want a "fun" challenge? Level 23 can be finished with five left alive. Requires insane timing, but you can do the bottom half with two stones instead of three.
I loved this game, and I think some people are being too harsh about it. Had to use the walkthrough, couldn't figure out how to pry open that damaged container. But the puzzles were clever, and the alien language was a nice touch at the end. 5/5
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