Better known as the Blue Mosque—for the 20,000 blue tiles that line its domed ceiling—this 17th-century architectural masterpiece is Turkey’s crown jewel. Its soaring domes and slender minarets were commissioned by Sultan Ahmed I in 1609; the mosque was intended to exceed Hagia Sophia in grandeur and thus to prove the superiority of Islam to Byzantine Christianity. Whether it achieved the former aim is one of those eternal Bach-versus-Mozart debates; you’ll have to decide for yourself (no comment on the latter). Since this is a place of worship—not a museum—you’ll need to conduct yourself respectfully here. You must take off your shoes upon entering (you’ll be given a plastic bag in which to carry them), and women are expected to cover their arms and heads (if you don’t have a headscarf of your own, you’ll be handed one at the door).