I like the overall game design, an action-oriented Plants Versus Zombies style tower defense. It's fast paced and is a lot of fun.
Having just the two sound effects is a bit repetitive, especially without any music. Something to consider is to have a couple of similar sound effects that that get chosen at random instead of the same one every time.
The wave system is a good start but it's hard to rebuild as the wave is still going while that timer is running. Consider either making the rebuild timer longer to account for that or starting the timer as soon as the last zombie is offscreen.
This is a solid game with a lot of potential for growth, good job.