I like the graphics that you used and that they are in the same artstyle, as it makes things easier to read. From what I know tower defense games can be quite difficult to make, so congrats on tackling this type of game!
I think adding a tutorial would really help people better understand what they have to do. I know from experience that it's not quite the easiest thing to implement. After all, after you've been working on the game for some time everything seems to make perfect sense to you, but for someone who first sees your game things can get confusing really fast. Good job!