Extremely effective, because although they restricted visibility, they gave the occupier almost unlimited time to survey and take aim at their target, with almost no risk of being shot back at.
You actually stand inside the structure of an arrow slit. The floor slopes down, toward the opening, to afford the best view of the terrain below, and the chamfered walls mean you have a wide field of fire, even with a weapon like a crossbow (that most unskilled defenders would be using). A bow-shooter would gain as much as 20–30 degrees wider field of shot in the same opening, over a crossbow user, for instance.
The exterior view of the same sorts of arrow loops show that it would be near impossible to shoot back through them at the defender.
Credit : Quora
Picture Credit : Google