Hagia Sophia in Istanbul is an amazing historical structure renowned the world over. Built by Byzantine Emperor Justinian I, initially as a Christian cathedral called Megale Ekklesia, the structure was later converted into a mosque by Sultan Mehmed II following the city's conquest in 1453. Over the centuries since, it has gone through many alterations and features important to the Islamic architectural tradition have been constructed during the Ottoman era of ownership. It nowadays serves as a museum open to visitors from all walks of life, showing off the mosaic tiles, minarets, mihrab (niche facing Mecca), minbar (pulpit), the grand chandelier, and awe-inspiring dome that have made it admired by people from all over the world.

It's a classic stop on your Istanbul tour, it's not to be missed.