Book excerpt: Cathedrals and great churches are among the most iconic sights of the world's towns and cities. Visible from miles around, the cathedrals of Canterbury, St Paul's, Chartres and St Stephen's in Vienna dominate their skylines. Others surprise by their statistics: Salisbury has Britain's tallest spire, Wells the largest display of medieval sculptures in the world, while King's College Chapel in Cambridge boasts the largest fan vaulting in existence. Not all are ancient: Dresden's reconstructed Frauenkirche opened in 2005 and Gaudi's masterpiece in Barcelona is still under construction. Award-winning travel writer Sue Dobson gives us a highly personal tour of their highlights.