Very interesting, and very characteristic both of this time and of its art, is the construction and the external appearance of this arch. It consisted of a series of three ring courses one above another, each of them covered by a flat course. The lower ring of the outside is destroyed and has disappeared completely. The bricks of our arch are of the usual form, not wedge-shaped. The laying is so slightly radial that at the vertex an actual three-cornered gap remains filled in with chopped brick. The central bricks were covered with asphalt before being laid, the lower ones are laid in mud and asphalt. The inner imposts are bound together by clamps made of poplar wood soaked in asphalt on a system which can no longer be clearly worked out. The lower ring alone formed an actual arch, each of the two higher rings begin some courses higher than the last and follow only a part of the semicircle, thus forming a segment. They begin nevertheless with a brick laid horizontal and not sloping. It is obvious that the planning of this arch construction is very faulty and inconsistent in comparison with Roman stone vaulting. The wall stands throughout on a level foundation bed. On the outside it is perpendicular, but on the inside the courses recede a little one behind the other, causing a slight slope and rendering the walls somewhat thicker below than they are above. This batter of the walls never occurs in buildings that are indisputably of the time of Nebuchadnezzar. On to this old piece of wall, with its three towers to the north and the south, the later walls are built with grooved and tongued expansion joints (see p. 36), for which purpose the old wall was hacked out as far as necessary. The later wall is plain; it formed, however, only a foundation for the now destroyed upper part, which certainly must have been furnished with towers. By this new building the old wall appears to have been strengthened within as by a Kisu, to which the palace walls are closely fitted by means of plain expansion joints.