Ninjas are infantry units that are very effective against Heroes (explorers, for example) and mercenaries. Ninjas themselves are mercenaries, so are easily countered by spies or other ninjas. The Ninja can also use Stealth Mode, which can be detected by an Explorer or Town Center (or other units).
The Ninja differs from the Spy in that it is classified as a mercenary, in contrast to a Spy, making the Spy very effective against Ninjas. The Ninja also has slightly less speed, higher cost, much lower Line of Sight and much lower multipliers, except against Native Warriors, but it has more hit points and much more (siege) attack. Overall, Ninjas are more well-rounded than Spies, and more deadly against Native Warriors and mercenaries, with the same effectiveness against heroes, and a lesser proficiency in scouting.
The Ninja also has different accessibility compared to Spies, as Spies are always available from the Church/Mosque from the Colonial Age onward (except for the Dutch Church from the JapaneseConsulate), can be sent from the Home City by European civilizations, or 4 can be sent from the British once at the expense of, while the Ninja can sometimes be trained at the Saloon/Monastery, or sent with Home City shipments by certain civilizations.
Ninjas are very useful in the third and fourth scenarios of the Chinese campaign (by choosing Japanese Mercenaries at the Consulate) because most enemies on these maps (namely, Jaguar Warriors and Eagle Warriors) are classified as Native Warriors, which the Ninja has bonuses against.
The Ninjutsu-Zukai is identical to the Ninja, but are easier to access since they can be trained from the Japanese Consulate if Japanese Isolation is chosen. They start with +10% hit points and attack than the regular Ninja, and are automatically upgraded in every Age starting in the Fortress Age (see here for the exact values).
"Shrouded by the mists of time, the exploits of the ninja continue to fire imaginations across the globe. During the turbulent years of Japan's sixth and seventh centuries, a secret class of warrior priests emerged. Rulers of opposing factions came to value the ninja for their skills in stealth, combat, and espionage.
As the centuries passed, the legendary powers of the ninja grew: invisibility, shape-shifting, flight; even the ability to pass through solid walls. Modern filmmakers blend historical fact with ancient myth to fashion a portrait of a warrior class that continues to fascinate and terrify."
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Infantry Breastplate (+10%)
Thin Red Line (+20%, British only)
Corselet (+25%, Spanish only)
Cree Tanning (+5%)
Maya Cotton Armor (+20%)
Navajo Weaving (+5%)
Carib Kasiri Beer (+10%)
Mapuche Tactics (+50% siege attack)
Zapotec Cult of the Dead (+20%)
Yoga (+5%)
Master Lessons (+10%)
Military Drummers (+10%)
Tilly's Discipline (+20%, Germans only)
Incan Road-building (+20%)
Apache Endurance (+5%)
Town Watch (+2)
Standing Army (-25%)
Incan Chasquis Messengers (-25%)
Mapuche Ad-mapu (-10% coin cost)
Coffee Trade (-10% speed, Dutch only)
Fencing School
Western Reforms
Acupuncture
TEAM Infantry Hitpoints
Hire Japanese Ninja
Improved Mercenaries
Dravidian Martial Arts
TEAM Shivaji's Tactics
TEAM Scout Infantry
Onin War
Hire Ninja Assassins
TEAM 10 Ninja Assassins
Ransack
TEAM Dueling School
TEAM Hand Infantry Attack
TEAM Inquisition
