Firstly, Castle Fight uses triggers to make units cast their abilities.
There is another, easier way to do it - just set the computer players in the game lobby, and they will automatically use spells based on standard abilities. For example, Blizzard will be cast if there are 2 or more units near a point, Curse will be cast when an enemy attacks a unit, Inner Fire or Bloodlust will be cast when a friendly unit attacks, and so on.