Starting at Plaza de Regla, 24003 León, Spain, open the Explorial app and enter the 6-digit game code that you received by text and Viator chat message. You begin in the shadow of one of the great Gothic cathedrals of Europe. The first landmark is the Catedral de León itself, whose 1,800 square metres of medieval stained glass — more than any other Gothic cathedral in the world — fill the interior with shifting coloured light at every hour of the day. From there the app takes you to Plaza San Isidoro, home to the Basilica of San Isidoro — a Romanesque masterpiece whose Royal Pantheon is decorated with 12th-century frescoes so vivid and complete they have been called the Sistine Chapel of Romanesque art. The route concludes at the Convento de San Marcos, a former pilgrim hospital turned Renaissance monastery whose elaborate plateresque facade stretches along the riverbank in a display of 16th-century architectural ambition. Around 2 hours on foot. León is best walked slowly — almost every building has something to say.