Towers can be constructed at any point in a Mission, whether at the start or in the middle of a wave.
Building Towers costs Elixir and each Tower has a various expense.
Try and make choke points with your warriors.
Save your super special moves for when you're in a real tight spot.
Use your hero's move when it's going to give you the biggest advantage.
Heroes are basically moving towers efficient in stalling creeps while outputting an outstanding amount of damage.
Every Hero has a Hero Power. If they have a legitimate target in range of the Hero, these can be triggered by tapping their button at the bottom left of the screen and will just trigger.