One of Istanbul's oldest churches, later turned mosque, is an easy ten-minute walk from the Hippodrome and its namesake, the Hagia Sophia. Construction on the Little Hagia Sophia (or Küçük Ayasofya in Turkish) began in 527 AD and was completed in 536, predating the Hagia Sophia, which was built from 532 to 537, by just a few years. The church, dedicated to Saints Sergius and Bacchus, has often been described as a trial run for the grand building up the hill, though this perhaps overstates the case. While both buildings are believed to be by the same architects and have a similar open plan under soaring domes, there are significant differences in their structures. Regardless of debates about its architectural pedigree, the building, which was converted to a mosque at the beginning of the 16th century, provides an opportunity to admire Byzantine architecture without the crowds. Don't miss the gorgeous marble columns on the ground floor and the second-floor gallery.