The Official Game Tutorial:
//Please Read this to understand the game//
Go through the levels and defeat enemies. Enemies starts to vibrate before they attack. After they have vibrated for about half a second they will attack you. When they are attacking they have a red electric particle effect around them.
The red bar is your health (slowly recharges after 7 seconds).
The orange bar is how much attack power you have left (slowly recharges).
The yellow bar at the top of the sceen is your xp progress. The text under is your level.
Your level amount is equal to the amount of followers you can have.
Followers make you stonger (More health and damage). If you lose your followers you lose your strength. Followers spawn every 4 second if you are not already on max (your level).