The Defender is a strong defensive unit that can be trained after researching the Shieldstechnology. As their name implies, defenders have good defense while lacking in attack and movement. The Quetzali tribe starts the game with a Defender.
Defending cities. Full-health defenders cannot be one-hit by anything, so that naturally makes them good for holding a certain point.
Holding choke points. Barring a sea invasion, if you put defenders across a point you don't want someone to reach, the enemy can't get through it, and even if they do, their army can't flood through it easily.
Countering attacks. Unlike other units, defenders have very high counter damage. Since most enemies can't resist attacking a defender, one of these units can thin out overly ambitious neighbors in the early rounds.
Defending against weak swarms. In a city wall, a defender has so much defence that damaged Warriors and Riders will actually deal 0 damage to it, and the defender's retaliation will instantly kill the assailants.
Ship swarm. On a naval unit, all the original unit's stats are ignored except for the health of that unit. And since the Defender is the cheapest unit with 15 health besides Polytaurs, you can just swarm the sea with them, quickly and cheaply securing naval dominance.
Weaknesses:
Defending against mobs of strong units. A mob of swordsmen or knights can take down a defender, although they will suffer some damage.
Fighting ranged units.Archers can continually move back and snipe it, ships can shoot from off the coast, and catapults deal too much damage for defenders to even be able to scratch it.
Strategy
If the AI starts spamming tons of defenders, build a wall of Warriors (or Swordsmen if you have them) and block the defenders but do not attack. Then, just wait for the defenders to attack you. Since defenders have low attack, they take a lot of damage when they go on the offense. The next turn, warriors can kill defenders with 5 or lower health.
Defenders can also be dealt with using ranged attacks, as their high defense means nothing if they cannot retaliate.
Trivia
The old Imperius Defender
Previously, Defenders had a black line on the shield, and a sword at their side.
