Little Hagia Sophia
Formerly known as the Church of the Saints Sergius and Bacchus, Little Hagia Sophia was constructed between 527 and 536AD by the Emperor Justinian. Supposedly Sergius and Bacchus had visited Justinian's uncle, Justin I, and told him Justinian did not pose any threat, thus sparing the future emperor's life. The emperor built the church upon his accession to commemorate the occasion.
As with many of Istanbul's former churches, it was converted to a mosque in the sixteenth century. Earthquakes and humidity have taken their toll on Little Hagia Sophia, however. Many of its treasures have been damaged or destroyed. A massive renovation was completed in 2006 and the former church was only then reopened to the public.
Little Hagia Sophia was, as its name might suggest, the precursor to Hagia Sophia. The design of the larger church was largely based on this one. The interior was once covered in marble and mosaics, though little evidence remains. The floorplan was borrowed from the basilica of San Vitale in Ravenna, Italy. The main hall is topped by a large dome. The arcade out front is home to a fountain, cafes, bookshops, and calligraphy boutiques.
It may be small, and somewhat redundant if you've already been to Hagia Sophia, but Little Hagia Sophia is worth a visit. Though it lacks the adornments found in other Byzantine churches, the atmosphere is pleasant, amplified by the natural light that bathes the interior. Remember that this is an active mosque, so be sure to be respectful.
Location: Küçük Ayasofya Caddesi