I did some balancing, found out that making the shield more expensive felt a lot better. Now it feels more strategic

I added three new cards at the moment, and one of them features completely new mechanic, one of the new cards costs 0 time. Which means it's a no brainer, just play it. But it do has some nice synergy. 

the other one is more special, its a card that targets every single enemy in the combat. 

Very soon there would be a new update with about 10 different cards that some of them would feature this new mechanic.