Game mechanics

The levels will be filled up with enemies that will chase you with the intent to kill you.
You can hide behind objects so that they cannot see you. Pay attention to the cone that represents the view field of your enemies: If an enemy spots you he will start to chase you and will alert his fellows. Every other enemy who is near enough to hear his alert will chase you as well.
The view cone of alerted enemies becomes red. As long as the cone is green, they don't know you're around.

The player is hiding behind a rock and can't be spotted by his enemy.

Even if an enemy can't see you, he could hear your shots or be alerted of your presence from their fellows. You can backstab your enemies with your bowie knife if you don't want to be heard.

Approach your enemies from behind to backstab them. Be fast, because they could hear you after a while.

Enemies could see you through fences, but they won't be able to shoot at you through them. You won't be able to shoot through fences as well, but there are weapons (dynamite and molotov) which can be thrown above these obstacles.

Launching dynatimet over a fence

When you have a gun or rifle equipped, around the target you will see a circle: that's the possible error for the current weapon. The higher is the precision of a gun, the smaller is the radius of this circle. If the circle becomes red, then the target is out of range and your bullet will disappear before reaching it.

A target at reach: the circle shows the error you could have shooting at it.

Every time you kill an enemy you will collect money that will be useful to buy new stuff at the end of each level.
You will find many civilians during your missions. These people aren't a threat to you and you shouldn't kill them. Obviously, they could be caught in the middle of a crossfire, but if you kill one of them you will be obliged to pay a fee. If you don't have enough money you will fail your mission.