"Agile. Deadly. Greedy. Ninjas steal Jade Coins and Essence from our opponents."
Ninja is the second troop you unlock in the game. It is unlocked after completing the "Ninja Stronghold" Mission in the single player campaign. They move fast and prioritize Essence Wells and Jade Markets, not attacking any other buildings until none of these remain. Ninjas do quadruple damage against Wells and Markets.
Though they deal relatively little damage per unit once all loot has been taken from the battlefield, en masse they can tear down buildings very quickly. Remember though, with low health they will fall to splash damage. Ninjas are extremely useful when protected by other units and left to steal all the resources in the enemy base.
At level 4, they begin carrying Ninja Stars into battle. They will throw these stars at whatever they are targeting (the crosshairs that appear when they are placed.) They normally throw these stars at resource buildings or walls, but placing several Star-throwing ninjas near a Jade Lion will swiftly destroy it. Likewise, any defensive troops will quickly die to ninja stars.
Some players think its one of the far weakest troops in the game, which is not true. Per troop size, the ninja actually does more damage than any other unit in the game unless you include splash damage in the calculation. Additionally, since they attack so quickly and move so quickly, very little damage and time are wasted between attacks, unlike the perpetually slow Oni Troll or the slow attack speed of the Fire Demon. Additionally, they make great additions to Commanders, since they will ignore orders and clean up whatever the commanders miss.