Simplify the following text for a Grade 13 native English-speaking person.
Original: The high walls and vaulted stone roofs were often supported by flying buttresses : half arches which transmit the outward thrust of the superstructure to supports or buttresses , often visible on the exterior of the building .
Simplified: The high walls and vaulted stone roofs were often supported by flying buttresses : half arches which transmit the outward thrust of the superstructure to supports or buttresses , usually on the outside of the building .