First off, really good job making a game all by yourself! I think you did a really good job for the time constraints.
I really like all the little animations you gave the enemy, hero and arrow sprites.
Balance and game design-wise, I think there is a lot to improve! I'm trying to think of what I would have changed without piling on too many new features. Let's see.
I think the game starts off a little too slow (especially if enemies spawn in the corners), enemies and hero walking speed could probably double.
You need a some more feedback when you damage an enemy to make it feel better. For example, when you click one, it should maybe squish a little bit. And maybe have a little sound (and very slightly randomize the pitch of the sound to avoid it feeling repetitive).
To make your ingame economy more interesting, you need more competing options for gaining power. Maybe you should be able to pick between a wizard and a knight. They could have varying things they bring to the table, maybe one is ranged but moves very slowly, and maybe the other is melee but fast.
I think you need some sort of hero upgrading, like movement speed or damage increases.
Maybe it would be fun if there is a faster enemy every once in a while.
I think Hero targeting is currently random, but I feel like that feels unfair to the player. Watching a hero walk from one corner to the other while surrounded by a gazillion monsters feels bad.
Something you can experiment with, and stuff people really seem to like and want in tower defense, is having LOTS of enemies. You want to feel like it's a flood of monsters coming at you, and you're awesome for stopping that. I think you should try to capture some of that fun. Obviously that would require heroes and click doing area damage instead maybe. But it's an idea!
Okay that's all my notes, I have to get going. Good luck.