At intervals along the crude-brick wall were towers projecting slightly beyond each face.[25] Only the bases of the towers have been preserved, so that any restoration of their upper structure must rest on pure conjecture. But, as rubble still fills the space between the two walls of burnt and unburnt brick, it may be presumed that the filling was continued up to the crown of the outer wall. It is possible that the inner wall of crude brick was raised to a greater height and formed a curtain between each pair of towers. But even so, the clear space in front, consisting of the rubble filling and the burnt-brick wall, formed a broad roadway nearly twenty metres in breadth, which extended right round the city along the top of the wall. On this point the excavations have fully substantiated the account given by Herodotus, who states that "on the top, along the edges of the wall, they constructed buildings of a single chamber facing one another, leaving between them room for a four-horse chariot to turn."[26] Even if smaller towers were built upon the outer edge, there would have been fully enough space to drive a team of four horses abreast along the wall, and in the intervals between the towers two such chariots might easily have passed each other. It has been acutely noted that this design of the wall was not only of protection by reason of its size, but was also of great strategic value; for it enabled the defence to move its forces with great speed from one point to another, wherever the attack at the moment might be pressed.[27] In fact it is only in the matter of size and extent that the description given by Herodotus of the walls of Babylon is to be discounted; and those are just the sort of details that an ancient traveller would accept without question from his local guide. His total number for the city-gates is also no doubt excessive,[28] but his description of the wall itself as built of burnt-brick tallies exactly with the construction of its outer face, which would have been the only portion visible to any one passing outside the city. Moreover, in one portion of the wall, as reconstructed by Nebuchadnezzar, its inner as well as its outer half appears to have been formed of burnt-brick. This is the small rectangular extension, which Nebuchadnezzar threw out to protect his later citadel now covered by the mound known as Babil.[29]