Hey, don't get me wrong, you have a solid base for a tower defense! I believe you're missing a twist but besides that your tower defense is has a lot of content, which is very nice!
When I say VFX and particles, I'm mostly talking about game-feel. I would add some screen shake, particles where the bullets hit, and more sounds in general: music, ambiance, SFX for the towers and enemies, etc. You probably already know him, but a good introduction to game-feel is this video from GMTK.
Cheers,
Gaba