I have finally got my head round the advances in architecture that are called "Gothic". The ceilings became made of stone vaulting. This made the roofs much heavier. The middle pulls downwards and pushes the edges of the roof outwards. This would cause the walls to be pushed out. So something is needed to resist this outward push. Hence the flying buttresses on the upper level. The walls on the Gothic cathedrals also have large windows which weakens them.
