Shadow Double Ability is the bonus ability acquired from the third (fourth on Mobile) upgrade of the Ninja Academy Specialty Building. Using this ability creates a shadow ninja identical in upgrades that is randomly positioned within the Ninja Monkey's range. However in the mobile version, a basic unupgraded shadow ninja is replicated. This shadow ninja only lasts for a short time, lasting 15 seconds. It has a cooldown of 120 seconds.
- Shadow ninjas can also be clicked to see their info and set their target priority. They can also be sold, however they give no cash.
- In Co-op Mode, the player is also able to click their partner's shadow ninjas, and do whatever they want with them.
- Selling shadow ninjas can bypass the max tower limit in Daily Challenges and the There Can Be Only One and Full House special missions, allowing one more tower, due to a glitch.
- In the Short-Lived special mission, if the player clicks on a shadow ninja, they can read "und" along with a letter partially cut off (probably an E) in the place where there originally was the sell button. It may probably mean "undefined".
- The sign won't disappear until clicking away from the shadow ninja, even if a round passes.
- If you sell the Ninja Monkey right after the Shadow Double Ability is used, the shadow ninja will remain for the duration of the ability (demonstrated in picture).
- The Shadow Double Ability is the only ability obtained from a specialty building, in the Flash version.
- However, in the Mobile version, the Shadow Double Ability is not the only ability obtainable via specialty building.
- The Shadow Double is shaped like a Dart Monkey from the first three BTD games.