The Inner Wall was the main bulwark of the capital. It stood on a higher level than the Outer Wall, and was, at the same time, loftier, thicker, and flanked by stronger towers. In construction it was a mass of concrete faced on both sides with blocks of limestone, squared and carefully fitted; while six brick courses, each containing five layers of bricks, were laid at intervals through the thickness of the wall to bind the structure more firmly. The wall rises some 30-½ feet above the present exterior ground-level, and about 40 feet above the level within the city, with a thickness varying from 15-½ feet near the base to 13-½ feet at the summit. The summit had along its outer edge a battlement, 4 feet 8 inches high, and was reached by flights of steps, placed generally beside the gates, and set at right angles to the wall, upon ramps of masonry. The ninety-six towers, now battered and ruined by weather, war, and earthquakes, which once guarded this wall, stood from 175 to 181 feet apart, and were from 57 to 60 feet high, with a projection of 18 to 34 feet. As many of them are reconstructions and belong to different periods, they exhibit various forms and different styles of workmanship. Most of them are square; others are hexagonal, or heptagonal, or octagonal. While their structure resembles that of the wall, they are nevertheless distinct buildings, in compliance with the rule laid down by military engineers, that a tower should not be bound in construction with the curtain of the wall behind it.[188] Thus two buildings differing in weight could settle at different rates without breaking apart along the line of junction. As an additional precaution a relieving arch was frequently inserted where the sides of the tower impinged on the wall.[189] A tower was usually divided by wooden or vaulted floors into two chambers. Towers with three chambers, like the Tower of Basil and Constantine at the southern extremity of the wall, and the Soulou Kaleh beside the Lycus, were rare. The lower chamber was entered from the city through a large archway. Occasionally, it communicated also with the terrace between the two walls by a postern, situated as a rule, for the sake of concealment or easier defence, at the angle formed by the tower and the curtain-wall. Upon these entrances the chamber depended for light and air, as its walls had few, if any, loopholes, lest the tower should be weakened where most exposed to missiles. Generally, the lower chamber had no means of communication with the story above it; at other times a circular aperture, about 7-½ feet in diameter, is found in the crown of the vaulted floor between the chambers. [Illustration: Portion of the Theodosian Walls (From Within the City).]