The almost circular building is the Church of Hagia Sophia (Holy Wisdom), erected in the period before the Ottomans captured Constantinople (1453). The double-light window on the north side is a feature of Byzantine architecture during the period of the Despotate of Epirus, a successor state of the Byzantine Empire established in the aftermath of the Fourth Crusade in 1204. Initially the church had a Greek-cross plan (a square central mass and four arms of equal length). It acquired its present round plan at a later date, while the interior has also been substantially altered (for the worse), with only fragments of the original frescoes surviving.