Pretty solid tower defense game. Difficulty scales a bit too fast for me. As some people said on the comments, there are some levels in which stacking towers (the main mechanic here) is not essential, or is not profitable. Feedback for the player is really well done, with sound and with effects, but i would add 1 thing (said on the bullet points below).
Things I would improve:
-Soundtrack that gives a bit more of a dynamic feeling to the game
-When you combine two towers, instead of the normal "placing" sound, another sound, so you know you combined two towers.
-I would make it so a combined tower shoots faster or something, so it feels more rewarding to combine towers.
-Maybe limit where turrets should be placed a bit more, so it gives players a bit more of a restriction and you will be able to "balance" a bit more the game mechanics and levels
Overall a pretty solid game, one of the best ones i've played so far for sure.
GGs!