"he gravity change can be accomplished only if there is a ground at reasonable height above the player?"

Correct!

"We should look into having some sort of spell inventory or similar thing in the game."

Thanks for the clip, the system looks nice. We could have a three(?)-slot-inventory at the top of the screen, too. When you need to switch to a different potion, you could hold down the potion key for a second and select a different one with the left/ right keys. I think we should not use the mouse as a control input.

What does George think?