“I invite many Americans and Europeans to my house to show them Turkish hospitality,” says Ender Mermerci. Her 300-year-old summer residence, called a yali, is adorned with artistic masterpieces and antiques. Built for great dignitaries of the Ottoman Empire, many such multi-million-dollar mansions line the shores of the Bosporus, owned by the city’s elite. Graciousness also abounds on the humbler doorsteps of Istanbul, where a visit from a guest brings offerings of tea and conversation.