In order to understand these dispositions and the operations of the siege which had now begun it is necessary to take account of the topography of the city. Constantinople in modern times comprises not only Stamboul but the large and even more populous district situated on the northern shore of the Golden Horn. This district was known in mediaeval times as Pera.[238] On the slope of Pera hill towards the Horn the Genoese were in possession of a walled city called Galata. Sometimes this city is described as Galata of Pera. In modern times, however, Pera is the name of the city on the north of the Golden Horn, exclusive of Galata. In 1453 what is now known as Stamboul was the only portion of the present city to which the name Constantinople was applied.[239] The city about to be besieged is situated on a peninsula at the south-west extremity of the Bosporus. It is, roughly speaking, an isosceles triangle with its base to landward. One of the sides is bounded by the Marmora and the other by the Golden Horn. It was surrounded by walls, which, with a few short intervals, still remain. On the two sides bounded by the sea they were built close to the water’s edge. In the course of centuries the Golden Horn had silted up a deposit of mud which even before 1453 formed a foreshore outside the north walls of a sufficient extent to have allowed Cantacuzenus to open a foss from Seraglio Point to Aivan Serai, formerly known as Cynegion. The side of the triangle most open to attack was that which faced the land and extended from the Horn to the Marmora. The walls on this landward side, constructed mainly during the reign of Theodosius the Second, had proved themselves during a period of a thousand years sufficiently strong to have enabled the citizens successfully to resist upwards of twenty sieges, and previous to the introduction of cannon were justly regarded as invulnerable.[240] The landward walls are four miles long. From the Marmora to a point where the land has a steep slope for about half a mile down to the Golden Horn, they are triple. The inner and loftiest is about forty feet high and is strengthened by towers sixty feet high along its whole length and distant from each other usually about one hundred and eighty feet. Outside this wall is a second, about twenty-five feet high, with towers similar to though smaller than those along the inner wall. This wall alone is of a strength that in any other mediaeval city would have been considered efficient.