Upgrade your Heroes (during battle too!)
Leveling up your heroes is vital if you want to make sure that you can keep up with the growing numbers of zombies attacking your barricades, but fortunately you can upgrade them during the battle as well.
As well as you have the money for an upgrade, you can simply tap the hero icon at the bottom and level them up. Of course, you can plan things out a little better and see what effect each of the upgrade has by leveling them up from the main menu.
When it comes to leveling them up, you should always try and have them all upgraded to the same level instead of focusing on turning a single hero into a beast and ignoring the others.