Under these conditions the vault was supreme in certain parts of the building. Its use was there so constant as to have almost the character of an unvarying law. Every palace was pierced in its substructure by drains that carried the rain water and the general waste from the large population by which it was inhabited down into the neighbouring river, and nearly all these drains were vaulted. And it must not be supposed that the architect deliberately hid his vaults and arches, or that he only used them in those parts of his buildings where they were concealed and lost in their surroundings; they occur, also, upon the most careful and elaborate facades. The gates of cities, of palaces and temples, of most buildings, in fact, that have any monumental character, are crowned by an arch, the curve of which is accentuated by a brilliantly coloured soffit. This arch is continued as a barrel vault for the whole length of the passage leading into the interior, and these passages are sometimes very long. Vaults would also, in all probability, have been found over those narrow chambers that are so numerous in Assyrian palaces were it not for the universal ruin that has overtaken their superstructures. Finally, certain square rooms have been discovered which must have been covered with vaults in the shape of more or less flattened domes. [Illustration: FIG. 43.--View of a group of buildings; Kouyundjik; from Layard.] We must here call attention to the importance of a bas-relief belonging to the curious series of carved pictures in which Sennacherib caused the erection of his palace at Nineveh to be commemorated. Look well at this group of buildings, which seems to rise upon a platform at the foot of a hill shaded with cypresses and fruit-laden vines (see Fig. 43). The buildings on the right have flat roofs, those on the left, and they seem the most important, have, some hemispherical cupolas, and some tall domes approaching cones in shape. These same forms are still in use over all that country, not only for public buildings like baths and mosques, but even here and there for the humblest domestic structures. Travellers have been often surprised at encountering, in many of the villages of Upper Syria and Mesopotamia, peasants' houses with sugar-loaf roofs like these.[163]