Wow, someone actually noticed this s**t.
Jokes apart, I realized the arrow key thing only when I actually published the game, unfortunately I don't know how to change that on Construct 3 yet. I'll probably will for the next game.
About the upgrades, yes, I should had done something more like a pop-up menu or at least write what they do.
The upgrades work that they spawn at the center (a gear and a "magic green fire thing"), one is for tecnologic and the other one for magic. When you collect one, it's shown one of the two low wall, that's probably the one you say spawned in the wall.
Considering that I worked for like 20-22 hours in total out of the entire week, I think too it's a good start.
(Fun fact: I think the best thing it's the art of the ghost; I don't know why, but I really like it)