Energy

Energy is used to build and upgrade your towers in the middle of battle. You'll have a decent amount of energy when starting a mission, enough for you to build a good line of defense early in the game. Killing different creeps provide energy that will sustain your army in the entire battle. Calling in the next wave of enemies will give you bonus energy but at the same time, will potentially make your job a lot harder. Do this only after weighing the pros and cons of your current situation.