Thanks for playing and giving feedback! I really appreciate it.
If you're struggling around waves 6-7, I'd recommend to focus on merging 3 towers of the same type together. The sum is greater than its parts!
Usually, you can see an example of merging towers in the "How to Play" section of the game and using the arrows on the left and right of the image to cycle through the slides, but I'll just post the example gif here. 
Thanks again!