I really like the zombie tower defence idea...cool setting and atmosphere, especially with the day-night cicle! I have to use the Godot lighting in future, it's so powerful.
My first thoughts what you could do...
Make more ways where the zombies can approach. And the ways should open step by step, so the thrill raises.
The sounds would need more variety. You could randomly change the pitch of each sound when it is played. What I did is to pick one random of multiple similar sounds what gives a better dynamic.
And you could give all characters (your own and enemies) different skins randomly.