One of the most beautiful things I saw in Istanbul was the Hagia Sophia. This amazing church was dedicated in 360 and served as a Christian Church until 1453 when it was converted into a Mosque. In 1931 it was secularized and in 1935 opened as a museum. Their are remnants present from all its former incarnations, unfortunately everything is in sad disrepair.
A few of the original stunning mosaics have been uncovered but all of them are extensively damaged. Though this World Heritage Site is one of the most visited locations in Turkey very little seems to be put into restoration and preservation.
Even considering the neglect it remains a stunning piece of history and architecture with astounding views looking int and out.