This is the original Cathedral, the original part of the construction of most of these large sites.  Usually the King built the Cathedral to celebrate some victory or other event.  Then the church, over the next several hundred years, kept adding to and remodeling until you had these massive sites housing hundreds to thousands of folks.  The roles for the sites expanded to protection, farming, training, arts, crafts, selling burial plots to the rich, etc.

You can see, from the scale, that the ceiling of the main nave has to be at least 30m high, towering as it does over the 2nd floor of the other buildings.  To build these massive places out of hand-chiseled stone is impressive.