Building a cathedral was an extremely complex and costly task.
As mentioned, Notre Dame took 200 years to complete.
One of the most complex parts of the construction was the nave.
The word nave comes from the Latin for "ship".
The word comes from the fact that the Church saw it as its role to help steer the congregation to salvation.
While architects had known for centuries how to build rudimentary arched ceilings, these were very weak structures.
The walls needed to be extra thick to withstand the pressure, as the ceiling pushed downwards and the supporting walls pushed outwards.
This meant that arched buildings were extremely low and dark, as any window would have weakened the strength of the walls.
In the 11th century, architects discovered a new technique, the rib vault, created by intersecting 2-3 arches.
With this system, pressure was diverted to the 4 supporting pillars and not the walls themselves.
Another system, flying buttresses - stone arcs connecting the nave to the outside walls - helped to relieve the pressure.
As walls became more stable, architects could build them higher and thinner and start to include massive stained glass windows, lighting up the cathedrals.