Sultanahmet: Hagia Sophia, Blue Mosque, Basilica Cistern, Istanbul
Concierge.com's insider take:
Don't even dream of leaving Istanbul without visiting this holy trinity of sights. Packed with tourists, touts, and queues they may be, but the history and scale of these buildings render them unmissable. The oldest of the three is the Hagia Sophia, built as a Byzantine cathedral by Emperor Justinian I nearly 1,500 years ago and converted into a mosque after the Ottoman conquest of Constantinople in 1453. Don't be fooled by its dull brick exterior: Its soaring dome and levhas (giant circular plaques inscribed in Arabic) are breathtaking, as are what's left of the original mosaics upstairs on the balcony level. Urban legend has it that the Hagia Sophia might not withstand Istanbul's supposedly "imminent" huge earthquake, so get there while you can! (Open Tues–Sun 9:30–4:30; open daily.)
Directly opposite the park outside is the majestic Blue Mosque (pictured), also known as the Sultanahmet Mosque, a relatively more recent structure built in the 17th century and still in use for worshippers at ordained prayer times, during which visitors are not allowed inside. This resplendent structure is one of Istanbul's finest, with its stacked shining gray domes and piercing minarets that are magically circumscribed by flocks of seagulls by night. Women should cover their heads when entering all mosques, although the authorities at this one seem to be a little more relaxed on that rule (open daily).
Finally, head down to the Basilica Cistern (Yerebatan Sarayı), which is smaller than the other two but no less incredible. The vast colonnaded underground well was built by the ancient Romans to store water that came into the city via the 12-mile-long aqueducts from Belgrade Forest. The cistern lay disused for centuries and has now been renovated into a superbly lit, slightly macabre subterranean wonder (open daily 9–5).