The first iteration of Hagia Sophia was built in 306AD to serve as Constantinople's Cathedral, but burned down in 404. Version two also burned down too, though several marble slabs remain. Construction on the current version began in 532 and lasted five years. It was built during the apogee of the Byzantine Empire under Emperor Justinian and you can really tell they went all out. This is one of the world's great buildings.
In 1204, crusaders captured Constantinople and in their zeal ransacked and desecrated Hagia Sophia. Due to the neglect of the European occupiers, it fell into disrepair and would remain closed until renovations were completed in 1354. After Sultan Mehmed conquered Constantinople in 1453, it was converted into a mosque, known as the Ayasofia Mosque. They removed the Christian vestments, and added the four minarets. It remained a mosque such until 1943, when Mustafa Kemal Ataturk converted it into a museum.
The most notable feature is the dome. The original was actually destroyed in an earthquake in 558, but was rebuilt bigger and better. It measures 31m in diameter and rises 55m. The interior is surprisingly colorful. Hagia Sophia also has some of the world's most beautiful mosaics. Many of them were plastered over when it was converted into a mosque, but have been painstakingly restored in recent years. A couple highlights include the geometric patterns in the upper imperial gallery and the Imperial gate mosaics. The other adornments are beautiful too, including two giant marble urns and other spectacular marble works.
Hagia Sofia is Istanbul's most popular tourist attraction and is definitely not to be missed. This is the epitome of Byzantine architecture, and it ranks up there with the world's great buildings. It also is a showcase of Istanbul's history, as there is an eclectic mix of Christian and Muslim artifacts. Hagia Sophia is easily accessible by public transportation. The T1 tramline stops just out front.
Location: Sultanahmet, Eminonu District
Hours: Open Tuesday through Sunday 09:30-16:30; closed Monday.