The key to being a good assassin is maintaining stealth. Staying stealthy isn’t as easy as it sounds, as there are a ton of factors that play into your ability to remain unseen. Stealth Mode can be toggled on or off by pressing 
/ 
.
Stealth Ring
The Stealth Ring appears if you are in stealth mode and anonymous. It shows the direction of nearby enemies and if they are above or below you.
Threat Ring
The Threat Ring becomes visible when enemies detect you. Each enemy is represented by an arrow on the Threat Ring and the color of the arrow shows the alertness level of the enemy, from white, to yellow, to red.
Detection
If you are in the line of sight of an alerted foe, they spot you. Enemies, such as Templars and Blighters, always notice you, even if they weren’t in an alert state. A detection icon appears above an enemy’s head to let you know that they can see you. The longer you are in sight, the more it begins to fill, moving from gray, to yellow, to red, signaling their level of aggression. While the grey gauge fills, they pose no threat. When it fills with yellow, enemies are more suspicious and physically investigate the target. Once the gauge is filled with red, the enemy comes after you, looking for a fight. Protectors, such as police and royal guards, only acquire you as a target if you perform an illegal action.
In Restricted Areas (displayed on the Mini-Map as red), enemies spot you much more quickly. The closer the enemy is, the faster the detection meter builds. Avoid contact with the enemy by breaking the line of sight before the detection meter fills all the way with red. In Restricted Areas, the gauge starts already filled with yellow, so you don’t have a lot of leeway before the enemies move to attack. Use stealth mode to evade enemy detection. When their gauge is fully red, they attack.
Last Known Position
Once the Detection icon fills completely with yellow, enemies investigate your last known position. If you move, and interrupt their line of sight, you leave behind a last known position ghost, and enemies move to investigate it. However, if you are already in conflict with an enemy, breaking line of sight may not solve your problem. The enemies search more aggressively, and aren’t always fooled by your last known position.
Enemy Awareness
When you near an enemy, they cycle through a few levels of awareness, shifting from unaware, to suspicious, and then to combat.
Suspicious State
As described under Detection, enemies become aware of you when you are in their line of sight. The longer you remain, the more aggressive they become. Enemies are not the only ones who can become suspicious of you. Protectors, such as police and royal guards, become suspicious when they see you perform any violent actions, such as shoving through civilians or wielding a weapon.
Conflict
If enemies are in conflict with you, the Mini-Map turns red until the conflict is resolved. Enemies chasing you have red icons over their heads.
Protector Investigations
When Protectors (for example, the London Police), represented by blue icons on your Mini-Map, perceive something suspicious without knowing the source, they investigate. While this is true of all enemies, Protectors are only interested in you if you seem suspicious. When Protectors investigate a dead body, they blame suspicious people close to their investigation area. Use this knowledge to create a diversion or throw a dead body near some enemies, disappear, and let the Protectors do the rest.
Vanishing
Being able to vanish is one of the most important skills in the game. Vanishing helps you avoid higher level enemies, as well as unnecessary fights in the streets.
Breaking the Line of Sight
To escape from conflict and vanish, you must break line of sight with all of your opponents. To do this, escape using alleys, enter a window, use the Rope Launcher to quickly reach a rooftop, or even use a smoke bomb. Once the Line of Sight is broken, the Last Known Position silhouette appears. That means it’s time to hide.
Hiding from Enemies
Once the Line of Sight is broken, enemies begin searching for you. Make sure you remain hidden until the search is over. You can keep moving to stay out of their line of sight, take cover behind a wall or behind a hiding door, etc., blend into the crowd, or even ascend to the rooftops. When the search is over, the Mini-Map returns to its usual color, and you have vanished successfully.
After Conflict
When you’ve successfully vanished, enemies return to their previous routine. However, they remember you and are suspicious of you in the future, which allows them to detect you more quickly.