Nice game, looks great. Like idea that first red monster is put into cage and shows player how he behaves. Also like in game text/tutorial.
Title screen - I thought that game is broken, because mouse clicking on "start" didn't work. It feels if mouse can be moved then clicking should be working also.
Jumping - character feels a little bit to heavy (falls to quickly), adjusting gravity would help.
Difficulty - I think punishing player when he basically completed level / got key is a little bit to harsh. (Tutorial level) I felt frustrated when i got key and fallen 3 times to the bottom, rage quit :D
I liked idea of moving platform in level 2, but I died quickly and had to restart game from Tutorial, It feels that player should restart level not whole game.
Because of this I didn't progress much.
How to implement "Saving" and "Loading" data or progress on Godot.
To have in game data storage use Singletons (AutoLoad) Basically, it is separate script that is attached to whole game "scene" and is always available.
To save/load data on game exit just write/read data to .txt file. Save on exit. load on enter. Docs tutorial.