MArcomage

There are plenty situations your next move is independent of the move of your opponent.
I hereby suggest a feature to automatically make your next move after your opponents move. As soon as you played your turn, the play buttons are replaced by autoplay buttons. If you click it, for a card, that card will automatically be played after your opponents turn. (clicking it again will undo it, clicking another card's button, will make that card the next played card instead.

With this I particularly have slow games in mind, which can be played faster this way. (If you don't have swift cards to play, of course.)

Wow. This is an excellent idea. It wouldn't always be useful, but for the times that it would be, it would be wonderful to have the option.

It is quite an interesting idea, however we'll always get the people who will arrange an automove and it will ruin their game (i.e. the opponent plays a killer card and you are left with 5 tower, and instead of playing a defensive card, you play Lake Fairy to increase your gems)

The same problem you have with normal moves: players make mistakes.
You already see players do stuff like you said all the time. This suggestion isn't gonna influence that for better or worse (unless you choose to play it while your opponent has quick/swift cards, but thats still a fault of the player, not the feature).

Simply put:
Players responsibility