Up The Security

Quality of Life
An indicator of the player gaining money perhaps just with a coin above their head to give the player a sense of when they are getting money. An indicator of the player taking damage and an indicator that the enemy takes damage would go a far way too. You wouldn't have to do much you'd only have to play a sound effect and maybe shake the screen.

Balance
The projectile speed of the bullets should be a little faster so they don't miss the enemy as often. Because of a combination of this and the slow reload speed the player often gets punished for no reason. 

Ideas
I really like the idea of the enemy getting stronger and the player having to remove pieces of armour to kill the enemies.

I think you could have added a range indicator and maybe some units with different effects, and upgrades to your existing units.

Art
The art is a simplistic style and works well for what your doing. It could've been a tiny bit better with some idle and animation frames but that could have taken too much time.

Sound and Music
There is no sound and music in the game and I think that would have really helped the game a lot. Without sound and music it just feels a bit bland.

All in all for your first tower defence you did a good job and I wish you a lot of luck in the jam!

uh instructions of how to play would be nice