This is decent. For a first game, it could be a whole lot worse. There are mostly a lot of small problems. For example, when going down the long vertical shaft, switching into a big wall causes the character to float upwards out of it, all the way back to the surface.
The graphics could use some work. Sometimes, the spikes are completely obscured by the background.
Losing all progress for what is quite an easy death seems a bit harsh.
I think I activated the doorway — it became white — but I couldn't go through. I ended up pressing Z by accident, which... restarts the game? That's an odd control.
Congrats on your first game!