Turn your back to the church and walk towards the stairway that you saw in the middle of the Stoa. Climb up the stairway and you’ll find yourself on the third level of the acropolis. When up, you’ll notice that in this part there is nothing standing. Look carefully at the ground. You will see that it actually designates a rectangular building with two uneven wings. You are actually standing inside the ghost of the Propylaea!