Waw amazing game!
There are a few issues though (just my opinion). I mean I wouldn't be able to create such amazing game myself, but if you want feedback you need to accept the negatives ones too
Let's categories the issues:
-Design wise, it needs improvement.
-People always have the habit to press 'Enter' after inputting their login credentials. It doesn't work here (part of your applet)
-A few design issues, text is not centered in your buttons.
-Try to keep the same flow in your menu design (if you center the text in buttons, use it in your options too) Or use something else instead of buttons.
-Where is your game title? except for the websites.
-What does the 0,75m mean on each of the selectable yellow cubes?
-As goes for the menu items: Try to keep your design pattern/flow the same in all levels:
-Some levels have too many 'simple' possibitilies. For example try to avoid this:
If you have portal2, Qube, Quantum... You'll see that none of them have such open small spaces
with ease access to the finish. This level can be a tutorial for a challenge though!
-There are a few level bugs, for example when I fall in the corner (as indicated on next image) I die but do get loaded into next level:
-I suppose this shouldnt happen? The red rectangle is where I can stand. the rest is plain texture, no blocks.
-As some of the members also stated: You have a chance to stand on Lava.
Please do not get me wrong with this post! Cause I really like your game and I'm soooo in love with your color pallet!
Seems weird but god some people have difficulties choosing the right color combination!