Didn't like it.
The gems count by 5 for no reason.
The art style makes it difficult to understand the layout of the map.
Losing all my upgrades on level 2 was annoying.
When I tried to do another upgrade on level 2 by placing the turret down, it didn't trigger the upgrade popup. So I started moving it down and spamming the spacebar so when it finally did open the menu. It did an upgrade on the first selection by default since its a very poorly thought out design.