Whenever the Byzantine Templars attacked an Assassins Den, the Assassins would engage in a Den Defense in order to maintain their control over it. Several such defenses were successfully carried out by the Italian Mentor, Ezio Auditore da Firenze during his stay in Constantinople.
During a Den Defense, Ezio had the ability to command and deploy Assassin units to defend the den from approaching Templar troops. Positioning themselves on rooftops and behind barricades, the Assassin forces would wait for the impending waves of Templars. Each Assassin had a combat specialty, ranging from crossbowmen to bombers, and barricades could also be placed to halt Templar progression. In order to call in assistance, morale was used to deploy Assassins and build barricades, which was gained automatically over time and sometimes from looting Templar corpses. Occasionally, deployed Assassins could be killed and barricades destroyed, which would prompt Ezio to call in replacement troops and construct new blockades.
He could also participate in the defense more directly, by firing his Hidden Gun at approaching soldiers, which would signal his troops to attack that squadron, or, periodically, command a cannon to be fired at a group of soldiers or a war machine.
The Templars would attack in several waves, leaving little time between each for the Assassins to prepare. From there, they would advance up the street, pausing only once encountering a barricade. On their death, Templar soldiers could be looted for akçe, bomb ingredients or morale points. However, once the soldiers reached the Assassin Den, they would begin attacking it, and would not stop until they forced the Assassins out or were killed.
List of Assassin units
- Assassin Leader: Leaders would not attack the Byzantine soldiers, but they were vital, as other Assassins weren't allowed onto the rooftops unless there was a leader there. Leaders would also increase morale over time.
- Barricade: Barricades would be placed in the streets to slow the Templars' progression to the Den. Assassins could also be deployed to barricades to help defend them. Typically, one barricade would be placed in front of the Den, as a precaution at the start of the battle.
- Greek Fire Barricade: A barricade mounting a flamethrower, with high damage and short range.
- Machine Gun Barricade: A barricade mounting a repeating gun with average range, low damage, and high rate of fire.
- Cannon Barricade: A barricade mounting a small cannon with long range and high damage, but had a very low rate of fire.
- Crossbowman: An Assassin wielding a crossbow. Possessed a high rate of fire, but dealt relatively low damage.
- Rifleman: An Assassin wielding a rifle. Dealt large amounts of damage, but had a lower rate of fire than crossbowmen.
- Air Assassin: An Assassin wielding a Hidden Blade. These Assassins would air assassinate Byzantine troops, though they couldn't attack again quickly, as they would have to climb back up to the rooftops.
- Bruiser: An Assassin wielding a medium weapon. Highly adept at melee, but had no ranged capability.
- Caltrop Bomber: An Assassin wielding caltrop bombs. While not dealing much damage, they would slow down Templars caught in its effect.
- Shrapnel Bomber: An Assassin wielding splinter bombs. They would deal damage in a large radius, but couldn't throw the bombs often.
List of Templar units
- Militia: Weak and poorly armored, their primary danger was in their numbers.
- Almogavars: Well-armored brutes carrying heavy weapons, though they would move slowly, making them easy targets.
- Riflemen: Equipped with a rifle, these Templars could return fire at rooftop Assassins, but had a low rate of fire and weak armor.
- Bombmen: Slow and poorly armored, but were equipped with bombs that would make short work of barricades.
- Scouts: Poorly armored, but were fast and could climb over barricades to attack defenders behind them.
- Infiltrators: Like scouts, they were poorly armored, but fast. However, they evaded defenders and would go straight towards the den.
- Stalkers: Highly dangerous in melee, they attacked Assassins on the rooftops instead of in the streets.
- War Machines: Heavily armored and slow, these were usually brought in at the very end of an assault.
- Ram: A heavily armored battering ram. It was limited to melee attacks, but dealt high amounts of damage.
- Greek Fire: A heavily armored platform with a flamethrower mounted on it. It dealt large amounts of damage near-constantly, but was limited to close range.
- Machine Gun: A heavily armored platform that fired projectiles over a long distance, dealing average damage. It was, however, ineffective against barricades.
- Cannon: A heavily armored platform with a mounted cannon. It had a very low rate of fire, but dealt extraordinary amounts of damage and had a long range.
- Successful Den Defenses counted toward the experience gained by recruits not currently away on contracts.
- Den Defenses noticeably got harder each time it happened, or when there were fewer dens immune to further attacks. Therefore it was advisable to keep Den Defenses to a minimum until all Den Leaders were Master Assassins.