Really nice visual style.
I like how the game just gets right into it, without unnecessary padding or filler content.
The game just.... starts!
Some people may not like that, but I like it a lot actually and found this a really strong point of this game - nice job!
I really thought the plants and the upgrades were really cool, the planting mechanics were very intuitive and easy to understand.
Only suggestion is I think the upgrades covering the screen means I think it should go ahead and pause the game until you select and upgrade instead of you keeping on moving in the background and the game still going, this might make it cleaner.
Other than that I think you did a great job on this game, it's really fun and it's a tower defense, something that may be hard to use in the same sentence for me since I might actually be a bit biased against these specific types of games, so you might have done really well on this one!