The art is pretty solid, and it’s great that the platformer has multiple playable checkpoints. The upside-down mechanic is a really cool idea as well.
Regarding the difficulty, I think I understand why some people mentioned it feeling frustrating. For example, the wall jump sometimes feels inconsistent. It seems like the player can wall jump as long as they are colliding with a wall while airborne and press jump, which can lead to accidental wall jumps or jumps in the wrong direction. One possible improvement could be requiring the player to face the wall before allowing the wall jump, so it feels more intentional.
There were also several moments where I got hit by offscreen enemies or projectiles, or fell off unseen cliffs. This might be improved by slightly panning the camera out more or giving the player better visibility of what’s ahead. It’s important that players can at least see hints of upcoming hazards to avoid feeling unfairly punished.
The checkpoints feel fine, and I don’t think simply adding more would solve the issue. The main challenge seems to come from the controls and camera. Right now, the difficulty sometimes feels more like memorizing patterns and timing rather than reacting with skill. In platformers, giving players enough space and visibility to react makes a big difference.
As an extra idea, you could consider allowing the player to temporarily pan the camera to preview what’s ahead before moving forward.
Overall, the core ideas are strong. With some refinement to the wall jump behavior and camera design, it could feel much more polished. I understand this is your first platformer, and you’ve done a solid job. Congrats on completing your first platform project!