MArcomage

Really interesting!
But I think it should be doing something in case there was not a tower attack. I wouldn't want to have a card, especially a rare one, in my hand and waiting for my opponent to attack my tower a reasonable amount (for its cost) to play it.

I'd suggest to put a default attack on it and bonus damage enemy tower in case they attacked your tower the previous turn.

I don't know how I feel about a card that says "If your opponent did X, you win regardless of everything else." It seems to me that there should be some condition on behalf of the player as well, or perhaps have it deal 2N damage if the existing condition is met. Survivable, but only barely. (The cost could then be lowered, of course.)