Many people have read the Dan Brown books. In his book, 'The Da Vinci Code' he talks of finding the 'keystone'.
Silas knew the information he had gleaned from his victims would come as a shock. "Teacher, all four confirmed the existence of the clef de voûte . . . the legendary keystone."
In architecture, a keystone is the stone at the top of an arch. It is the supporting element for the entire arch— without it the arch would collapse (in a cathedral dome the keystone holds the intersecting ribs).
In the first picture you can see a generic "true" stone arch above the door- the centre block being the 'keystone'.
I just love the colours and textures of this old place.