Steam Trading Cards for Yet Another Zombie Defense HD were just approved by Valve and they are live now! If you have spent enough time in the game you should have received them already.
And by the way - we've investigated the crashes in long multiplayer sessions and it looks that we've found what causes them. The crash logs some of you have sent us were really helpful and we are grateful for them! An update improving game's stability will be released tomorrow and we apologize for having this issue in the first place.