Which wall spells require anchoring on the ground?

Wall of Stone, cast in mid air. Does it stay there? Does the casting fail? Does it become Rain of Falling Rock?

What damage might that do?

Which walls can hang in the air? Wind? Force? Fire?

It would be up to the GM. Off the top of my head, I'd allow it to be cast in mid air and have it fall doing AoE damage vs. basic reflex of 5d6 bludgeoning (which is the same damage inflicted by a Wall of Fire heightened to 5th level). Then the area would be difficult terrain afterwards.

Something similar for Wall of Ice, except the difficult terrain would only last until the ice melted and it would do extra damage to creatures vulnerable to cold/ice.

Other walls I'd allow to hang in mid air since they aren't physical anyway.

Walls that don't explicitly state they can "hover" should be assumed to operate like real-world walls - from the floor/ground up.

And if a wall spell could be cast in the air, it definitely would not then fall and cause damage - that's definitely the kind of thing the spell would actually mention if it were possible.

It's important to remember that unless a concept has a new definition explicitly provided in the game rules, that the authors are using the general meaning of words that people use in day-to-day communication.

It's important to remember that unless a concept has a new definition explicitly provided in the game rules, that the authors are using the general meaning of words that people use in day-to-day communication.

Sure, but I don't know what that means in the context of a wall of wind, or wall of force. Iron Man seems to be able to "repulsor" things out of the air while Iron Man is flying.

It means that, unless the text actually says otherwise, "wall" means "wall, like all the ones buildings are made of, which go from the floor up"

...and Iron Man has nothing to do with this RPG. Bring him up when you're talking rules for GURPS or Hero System or something.