There should also be a way to take back which ability you choose. For example I was using the telekinetic wizard when my opponent attacked me and I accidently clicked the block button and I wasn't able to go back and activate his ability instead of blocking even though I haven't declared the blocker yet. So for those types of things you should be able to cancel before you declare something definitive.
There's a way to undo defending and paying energy costs now. For defending, there's a cancel button in the top left corner (where End Turn normally is). For paying energy costs, click the "X" in the top right corner of the paying energy cost dialog.
I can't add undos to every action, however, because traps and other abilities can occur in between. For example, if you could undo the playing of an Orb, this would prohibit traps that spring in response to playing an Orb, or abilities that draw you cards in response to playing an Orb, because it'd give you free information about the game state. Hope you guys understand why you can't undo just anything.
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 "Hands of War 2 - Expanded Edition"){kind=icon}
 "Hands of War"){kind=icon}
 "Orbs CCG"){kind=icon}
 "Dream World"){kind=icon}
There's a way to undo defending and paying energy costs now. For defending, there's a cancel button in the top left corner (where End Turn normally is). For paying energy costs, click the "X" in the top right corner of the paying energy cost dialog. I can't add undos to every action, however, because traps and other abilities can occur in between. For example, if you could undo the playing of an Orb, this would prohibit traps that spring in response to playing an Orb, or abilities that draw you cards in response to playing an Orb, because it'd give you free information about the game state. Hope you guys understand why you can't undo just anything.