The oldest building in Istanbul is the Hagia Sophia, which was initially built as a cathedral in the 6th century. The Hagia Sophia was commissioned by the Byzantine emperor Justinian I and was designed by the architects Anthemius of Tralles and Isidorus of Miletus. 

It was completed in 537 and was considered one of the most impressive and technologically advanced buildings of its time.

The Hagia Sophia is an architectural masterpiece, with a massive central dome supported by four piers and surrounded by smaller domes and semi-domes. It is decorated with intricate mosaics, marble columns, and gold leaf and is filled with works of art from the Byzantine and Ottoman periods.

After the fall of the Byzantine Empire, the Hagia Sophia was converted into a mosque by the Ottoman sultans and served as the primary mosque of Istanbul for almost 500 years. In 1935, the Turkish government converted it into a museum and is now a UNESCO World Heritage Site.