The tower N, an irregular quadrilateral building in two stories, measures 48 feet by 43 feet; the tower S, also quadrilateral, is 36 feet by 47 feet. But although closely associated, the two buildings differ greatly in style of construction. The masonry of N is irregular, having a large number of pillars inserted into it; often partially, so that many of them project like mock artillery. On the other hand, the tower S is carefully put together with the usual alternate courses of stone and brickwork, and is, moreover, ornamented with a string-course. A similar diversity of style is observable in the counter-fort. The portion about the tower N is built of small stones roughly joined, whereas the portion about the tower S consists of splendid large blocks, regularly hewn, and carefully fitted. Manifestly the towers are not the work of the same period. The tower N is commonly regarded as the tower of Isaac Angelus; while the tower S has been considered, since Dr. Paspates propounded the opinion, to be the Tower of Anemas,[518] which stood in the vicinity of the Palace of Blachernæ, and is famous in the annals of Constantinople as a prison for political offenders of high rank. The chambers in the body of the wall, behind and to the north of the towers, Dr. Paspates thinks, were the cells of that celebrated prison. How far these views are correct can be determined only after the towers and the chambers in the adjoining wall have been carefully surveyed. The plan attached to this chapter will render the survey easier and clearer.[519] At _x_ was a small arched postern, by which one entered the vaulted tunnel Z, that led through the counter-fort G´ to the gateway _l_ in the north-eastern side of the tower S. The sill of the postern _x_ is now nearly 10 feet above the exterior ground-level, but originally it was higher, so that persons could pass in and out only by means of a ladder that could be withdrawn at pleasure. The postern _x_, the tunnel Z, and the gateway _l_ are now built up with solid masonry to the spring of the vault, obliging the explorer to make his way on his hands and knees in a most uncomfortable manner.[520] Judging from the carefulness of the work, the passage was blocked before the Turkish Conquest. By the gateway _l_ one enters the lofty vestibule _b_, now in total darkness, so that all further exploration requires the aid of artificial light. The original floor of the vestibule is buried below a mass of earth which stands at the present level of Z and _l_. In the wall to the right is a low arched niche, _i_; in the wall _g_, directly in front of the explorer, a wide breach opens into E; while in the wall to the left is a loophole O, now on the level of the present floor of _b_.