You start this level on the roof of the cathedral. Take a few steps foward and climb down on the arche on your right. You'll see an enemy down below. Use your sniper rifle to take him out easily. Keep walking carefully and you will be attacked by a guard from behind a stained glass window. Kill him and make your way to that same window in order to enter the building. Come back in the corridor and go left to the third laboratory. Behind the glass doors are three scientists. Eliminate them and pick the keycard that one of them is carrying (checkpoint).