Can Wind Wall and Defy target any spell on the chain, or only the spell they are reacting to?

Ruling: Wind Wall and Defy can target any spell on the chain, not just the most recent one they are reacting to.

Nuances:

• Even if counter spells could only target the most recent spell, an opponent could wait until each spell resolves one by one and react when the desired spell becomes the topmost item on the chain
• There is a window to react after each resolution (except after the last non-showdown item)