The finest example of Byzantine architecture is the church of Saint Sophia (Holy Wisdom) in İstanbul, constructed by the emperor Justinian in the sixth century. Evidently, its structural design was something altogether new in the history of architecture. The central feature of the design was the application of the dome principle to a building of square shape. The church was designed in the form of a cross, with a magnificent dome over its central square. The main problem for the architects was how to fit the circumference of the dome to the square area it was supposed to cover. The solution was to have four great arches spring from pillars at the four corners of the square. The rim of the dome was then made to rest on the keystones of the arches, with the curved triangular spaces between the arches filled with masonry. The result was an architectural framework of marvellous strength, which at the same time made possible a style of imposing grandeur and delicacy. The dome itself has a diametre of 107 feet and rises to a height of nearly 180 feet from the floor. So many windows are placed around its rim that the dome appears to have no support at all but to be suspended in midair.