The circumference of the first fortifications of the city, built by Romulus, was thirteen miles. The greater part, however, of this large area was occupied by fields and gardens, which it was the object of the founder of the empire to preserve for arable purposes, from the incursions of the different enemies by whom he was threatened from without. As Rome gradually increased in size, its walls were progressively enlarged and altered by subsequent rulers. But it was not until the reign of the Emperor Aurelian (A.D. 270), that any extraordinary or important change was effected in the defences of the city. That potentate commenced the erection of walls, twenty-one miles in circumference, which were finally completed in the reign of Probus (A.D. 276), were restored by Belisarius (A.D. 537), and are to be seen in detached portions, in the fortifications of the modern city, to the present day. At the date of our story, then (A.D. 408), the walls remained precisely as they had been constructed in the reigns of Aurelian and Probus. They were for the most part made of brick; and in a few places, probably, a sort of soft sandstone might have been added to the pervading material. At several points in their circumference, and particularly in the part behind the Pincian Hill, these walls were built in arches, forming deep recesses, and occasionally disposed in double rows. The method of building employed in their erection, was generally that mentioned by Vitruvius, in whose time it originated, as 'opus reticulatum'. The 'opus reticulatum' was composed of small bricks (or stones) set together on their angles, instead of horizontally, and giving the surface of a wall the appearance of a sort of solid network. This was considered by some architects of antiquity a perishable mode of construction; and Vitruvius asserts that some buildings where he had seen it used, had fallen down. From the imperfect specimens of it which remain in modern times, it would be difficult to decide upon its merits. That it was assuredly insufficient to support the weight of the bank of the Pincian Mount, which rose immediately behind it, in the solitary spot described some pages back, is still made evident by the appearance of the wall at that part of the city, which remains in modern times bent out of the perpendicular, and cracked in some places almost from top to bottom. This ruin is now known to the present race of Italians, under the expressive title of 'Il Muro Torto' or, The Crooked Wall.