Congratulations on making a game! I liked the sounds!
Biggest of all, I think there should be a button to fast forward to the next wave if you have nothing summoned, and when I had 5 or 15 coins, I couldn't buy the units that cost 5 and 15 coins respectively. There's definitely a lot of bugs, but I waited all the way until wave 3. The biggest units that should cost 50 seemed to just cost 5 and be the smallest units (maybe that's because I didn't have 100 which I'm guessing is required). Also, the optimal strategy seems to be to spam units down the same line, so unfortunately it didn't add much to have two lanes (other than showing you could make it - kudos to that). I wonder what happens when the life gets down to 0, but it's too time consuming to figure it out, unfortunately.
And it's definitely not ideal that you have to install Game Maker to play the game, but I'm sure you know that.