Builders of gothic cathedrals reinforced the walls of their architecture from the outside with what?

I believe the correct answer is piers and flying buttresses.

Builders of Gothic cathedrals reinforced the walls of their architecture from the outside with piers (such as bridges or arcs) and flying buttresses (a type of buttress which is composed of an arched structure that extends from the upper portion of a wall to a pier) in order to provide the lateral support.