That was a really enjoyable defense game! I didn't expect the gameplay to have that much depth, between the quite big amount of energy required to buy things, making your purchases much more impactful, and having to get close to energy pickups to grab them (that specifically reminded me of Mann vs Machine, your game in co-op would be a blast!) A neat little game overall that could've benefited from some music and a better color palette - reading the green text over the yellow background of the main menu was hard!
PS: Was the shopkeeper inspired by Enter the Gungeon?