Once a church converted into a mosque, the Hagia Sophia is a treasure trove of Islamic and Christian artefacts. The exquisite interior surfaces are sheathed with fine marble and gold mosaics, while the exterior has plain stuccoed walls. Be inspired by the Islamic calligraphic roundels suspended from the main dome since the 19th century. Head for the ground level to see the divine Mihrab, made of excellent marble and facing the direction of Mecca. Do not miss the wonderful mosaics portraying the Virgin and Child and the Archangel Gabriel, but the best-known would likely still be the one depicting Christ Pantocrator, flanked by the Virgin Mary and John the Baptist. Before leaving Hagia Sophia, look out for the stone cannonballs in the courtyard, which were actually used by Mehmet the Conqueror during his victorious 1453 battle for Istanbul.