Today, the great cathedrals of Europe stand as beautiful, imposing monuments - the pride of parishioners and the wonder of tourists. It is difficult for us now, even with all our engineering and architectural skills, to imagine the extraordinary ways these medieval houses of worship were constructed.Midway through the twelfth century, the building of cathedrals became a crusade to erect awe-inspiring churches across Europe. In their zeal, bishops, monks, masons, and workmen created the architectural style known as Gothic, arguably Christianity's greatest contribution to the world's art and architecture. The style evolved slowly and almost accidentally as medieval artisans combined ingenuity, inspiration, and brute strength to create a fitting monument to their God.Here are the dramatic stories of the building of Saint-Denis, Notre Dame, Chartres, Reims, and other Gothic cathedrals.