it was hard for me to get it at first, once i did though it was a really fun experience. i thoroughly enjoyed it :D
the sfx were good, the music was good but i think maybe making it a little more desperate sounding could maybe be a good idea? i liked the art a lot its really good and you made several good decisions in relation to it in my opinion, the mechanics and gameplay loop was fun. i think that the towers were a little confusing as to what they did at times, my biggest recommendation is a tutorial embedded into the game itself (like as you play the game it tells you the tutorial).
edit: i also feel i should add that its kinda hard, i feel like it ramps up in difficulty very fast. i also couldnt see the stones i was collecting (currency) very well, maybe a outline could help with that?
its a really good idea though and a really good game inspired by pvz that takes a nice spin on it! good job!