This was very nice! It really makes me feel a great sense of nostalgia of that good old flash game era games! The gameplay is smooth, the upgrade is very balanced too! (it's rare to see such balancing in a jam) There are also different type of enemies when going further and further, the phoenixes and griffins are also a extremely nice touch too! Very well done!
I will add a bit feedback that it would be nice if the background changes later in the level, making the stage more interesting that what it currently is. Also some upbeat music please, that would be great too! There is a bug where the upgrade buttons are still active even though it's already upgraded to the highest level. (luckily pressing that don't consume gold) The upgrade scene can also benefit from adding icons instead of just words, but currently it's already quite decent.
Anyway, you done a great job on this game and you should be proud of yourself. Well done!