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As you progress, you'll find levels with thicker borders that need 2 hearts to play; complete that to get the hero card.
This is really a great game but I find the use of a timer a poor choice in a paid game; fair enough in F2P (and the other pay options are fine)...
I finished the missions today (top score is just over 400k) and found it quite frustrating that there is just a "more missions coming soon"...
