Wow this is impressive! The concept is really very clever!
I'll admit that at first I didn't know what I was looking at, but it was so obvious after clicking next wave.
I really like the design here.. its like an auto-battler tower defense type game. The coins' AI keeps the game interesting.
I did notice that some enemies seemed impossible to kill even with the emeralds, so loosing felt inevitable and a little forced or artificial. So the difficulty balance could stand to be adjusted in my opinion. But the skeleton of a great game is there!
The UI worked as intended.
The music is really good and didn't feel too repetitive. The sound effects worked well too!
Something that holds this game back gravely is the graphics, but not because of the art.. because of the settings. Unity by default tries to treat images like hi res graphics.. so when you import pixel art in Unity always be sure to set the texture settings' filter mode to "point" and set the compression setting to "none". You could even go a step further and put a pixel perfect component on your camera object. I hope this helps you on your journey!
This game definitely has potential and I could see many people enjoying a more balanced version with the graphics settings sorted out. Thank you so much for making and sharing this!