Tower defense is a tried and testing formula. Your version was pretty compelling. Good job.
More polish obviously includes sound.
I second the idea to include a fast forward button because at some points it's boring to wait when you know what's gonna happen.
In terms of theme - it's a relatively common interpretation in this jam to have upgrades when you die. To have downgrades when you win does a little something something; it becomes an auto balancing mechanism. Pretty interesting! But ultimately the only effect is I imagine that it averages out the amount of time it takes people to play.