Really fun game!
I liked how your health and currency (time) were combined into a single mechanic. Initially, I tried being dynamic and jump/move around, but I found that less movement works better for me. Eventually my upgrades became too powerful, and I didn't have to move at all. There were some performance issues at some point as too many projectiles were spawning, so I had to stop upgrading around wave 6.
The sound effects were very well coordinated with the soundtrack, especially the bell.
Art-wise, I think you did a good job at using colors to your advantage to make the player and mobs stand out from the dark buildings. If you jump too high (which I didn't do much personally), the bright sky can get in the way of seeing things properly. Many of the mobs only had an outline though. Filling them with solid color would help with the visibility. You also use quite many hues in the player/mobs/projectiles sprites which is inconsistency with your background. You could use a similar consistent palette that stands out from the background in some way, for example, using warm colors for the background and cold ones for the sprites (or dark vs light, etc). Some of your sprites could also use a bit more work, especially the main character. On the other hand, I thought your Skapler and Graphire mobs had a lot of personality.
Cheers!