|Explanation of cross-in-square from this fascinating site.|
It was built by Emperor Basil I the Macedonian (c.830 - 886). Although he started as a peasant, he advanced politically until he was in a position to usurp the throne of Emperor Michael III in 867. He set out to create a new golden age of Byzantine art, and he wound up being considered one of the greatest Byzantine emperors. In his desire to reproduce the glory of the reign of Justinian I, he started a building campaign. The pinnacle of this campaign was the Nea Ekklesia, which he considered his answer to the magnificent Hagia Sophia.
One of the things that made it "new" was the floor plan, something called "cross-in-square." Typical churches before that time—and, truthfully, after that time as well—were laid out like a cross, longer than they were wide. Nea Ekklesia broke that mold. Byzantine architecture had already shown a preference and flair for domes, and mounting them on a square base with a feature called a pendentive. Nea Ekklesia was a new style that filled out the cross shape by centering it in a square and putting several domes over the four additional sections. (See the illustration above for an example of a standard cross-in-square.)
As important as the Nea was, it was eventually turned into a monastery (called, perhaps predictably, "New Monastery") in the 11th century. After the Ottomans conquered the Byzantine Empire in 1453, the building was used to store gunpowder. It was destroyed in 1490 when it was struck by lightning.