Your towers can do physical and magical damage. Refer to the library to see which type is more effective against a particular enemy. When a new enemy appears, tap the small exclamation mark at the top right of the screen to view their information. Below their health will be a red or blue shield and the words "none", "low", "average", or "high". The red shield represents physical resistance and the blue shield is magic resistance. Its description shows its amount of resistance. If an enemy has high magic resistance, use physical damage towers and vice versa. Position slow and powerful towers near disabling towers. For example, partner the Apprentice tower which does large amount of magic damage but has a long cooldown with the Sleeping Lizard tower, which slows enemies.