Each Hero is trained in commanding a specific type of Unit. These Units are divided into three main class types:

Infantry - The Academy is where Infantry Units get training. If you are lucky, you will get Infantry Advancement Scrolls, which are needed to advance to a higher battle class.

Archery - The Academy is where Archery Units get training. If you are lucky, you will get Archery Advancement Scrolls, which are needed to advance to a higher battle class.

Mages - The Academy is where Mage Units get training. If you are lucky, you will get Mage Advancement Scrolls, which are needed to advance to a higher battle class.

As the level of Units Producing Buildings (Barracks, Shooting Range, Mage Tower) increases, new Units will become available for your Heroes to command. Heroes can only command Units they have had training for in Battle Classes. It is vital to continue to learn in Battle Classes to improve your Heroes for upcoming battles.

Visit Kingdom Hunter's Official Guide for more information about the game.