The zombies are out and they are not going to wait for you till you are ready. They would champ at you no matter what. Their primary purpose is not just to kill, but to eat and make more of themselves. In this game, you need to know who to upgrade since there are lots of enemies and only a handful of heroes to fight them. You need to choose, which ones you need to upgrade and fight them with. The game would be harder as you make some progress, so make sure that everything would be in your favor the first time.