One of the most incredible stories within these walls takes place in the Chapel of the Blessed Sacrament. Look for the magnificent silver altar, a masterpiece created in the late seventeenth century. In 1809, as Napoleon’s army marched into Porto, the locals knew the French soldiers would likely loot the cathedral for its precious metals. To save the treasure, a local sacristan or priest, according to legend, covered the entire silver altar in a thick layer of white plaster or paint, making it look like a cheap, ordinary wall. The French troops walked right past it, never suspecting that a fortune in silver was hidden just inches away. This cathedral has also witnessed major royal history; in 1387, King John the First of Portugal married the English princess Philippa of Lancaster here. Their wedding didn't just celebrate a marriage—it sealed the Anglo-Portuguese Alliance, which remains the oldest diplomatic treaty in the world still in force today.