Several of the secret achievements are simply your assassination targets!  No need trying to search for them.

If you kill all of the guards, you can speak to the citizen and gain a particular bonus. Usually a group of vigilantes will appear where you perform the rescue: these guys will harrass guards that become hostile to you,

If you’re cornered in a street fight, keep in mind that your best option, especially early in the game when you don’t have many combat maneuvers unlocked, is to run. Untarget your opponent, enter high profile mode, and run up the nearest wall to see if there’s a handhold; there almost always will be. If you can reach the rooftops, you’ll have a few seconds before your opponents find a way to reach you, giving you time to draw your blade and strike them as they run up a ladder, or to simply run away and try to find a rooftop garden to hide in.

Your synchronization bar will regenerate your health over time, even in combat. If you find yourself dangerously low on health, run away from your enemies for a bit to regain some health, or simply hide until it comes back.

When you attack an opponent, you can tap the attack button again when your sword hits your foe for a combo kill. If you time it right, your sword will attack once, then you’ll attack again immediately for an instant kill. The second attack will occur whether your opponent takes or blocks your first strike, but it won’t occur if they dodge it.

Holding down the attack button will cause your character to unleash a slow but powerful attack against your foes.

Low Profile assassinations are pretty easy to pull off on unsuspecting foes; simply get close and tap your button to take them down. This allows you to stealthily kill a target without alerting anyone nearby; even if a beggar is looking directly at you, they usually won’t notice that you’ve just killed someone.