Cancel, Rare, 20g
Negates all effects of opponent's last round
This needs more clarification, because as I see it it seems nearly pointless. By negating all effects, you are setting all game numbers (towers, walls, facilities, resources, tokens) back to what they were at the start of the opponent's turn and replacing whatever new draw and/or hand replacements with the cards that had been played and discarded. So you're out 20 gems and your opponent gets to take the exact same action again because whatever amount was spent playing the card has been restored. Granted, this can mess with Aria timing (I'm assuming it's not negating the round counter), and you can undo random draws that would have spelled your end had they remained in hand, but it just seems like it's delaying the inevitable.
Now if this isn't what was intended, the exact effects need to be spelled out, what is and is not being reverted. Just make sure it doesn't end up being near identical to an existing card.