The principal Mosque is also of peculiar interest, and presents an architectural problem which has never been quite fully solved. Two sides of its courtyard are formed by the façades of an ancient palace--a palace of regal dimensions, and constructed in a style that is admittedly unique. One of these façades is in two stories, with a pointed arcade below and square-headed windows over; the other has now but one story which consists of a pointed arcade.[20] These are not quite Romanesque in style, but more Romanesque than Oriental. They are rather like primitive versions of the Otto Heinrichs Bau at Heidelberg Schloss. But the building to which they are nearest akin is Diocletian's famous palace at Spalatro; albeit they are far less massive, and far more fantastically ornate. The theory most generally adopted concerning them is that they formed part of the palace of the Armenian king, Tiridates; and this theory is strongly supported by their resemblance to the palace at Spalatro, for Diocletian and Tiridates were contemporaries and close allies. Amida was one of the great fortresses that guarded the southern frontier of the Roman Empire. Northward, in Asia Minor, _Pax Romana_ had a fairly long innings; but Parthia and Persia to the southward were at no time definitely subdued. The hold of the Romans on Mesopotamia was indeed in some sort analogous to the hold of the Austrians on Italy previous to 1860. They regarded it as within their "Sphere of Influence," and sometimes they judged it expedient to "assert their interests" by invading it. But generally they found that enterprise was a bit beyond their capacity; their real "Scientific Frontier" lay along the mountains in the north. And here they, too, maintained their four great fortresses; not ranged in a square like the famous Austrian Quadrilateral, but _en échelon_ one behind the other along the southern slopes of the hills. Nisibis and Daras were in the forefront; Amida and Edessa withheld in reserve behind them. And though thus in the second rank, Amida got its full share of fighting when the kings of resuscitated Persia began to make invasions in their turn. Amida's defences were perfected, and its arsenal formed, by Constantius; and it was Constantius' great opponent Sapor II who undertook its first memorable siege. The great Sassanid Shah invaded the Roman territory with a huge army of 100,000 men in the year 360. He had at first intended to ignore the fortresses and to scour the hinterland for plunder; but as he rode past the walls of Amida an arrow struck his helmet, and he turned upon the place like an angry bull. His summons was answered by a volley from the _balistæ_ which slew the only son of his chief auxiliary, Grumbates the king of the Chionites; and Sapor swore to the bereaved father that he would not rest till he had taken the city in revenge.