5. All the essential materials for plain but durable constructions being thus procurable on the spot or in the immediate neighborhood, the next important point was the selection of proper sites for raising these constructions, which were to serve purposes of defence as well as of worship and royal majesty. A rocky eminence, inaccessible on one or several sides, or at least a hill, a knoll somewhat elevated above the surrounding plain, have usually been chosen wherever such existed. But this was not the case in Chaldea. There, as far as eye can see, not the slightest undulation breaks the dead flatness of the land. Yet there, more than anywhere else, an elevated position was desirable, if only as a protection from the unhealthy exhalations of a vast tract of swamps, and from the intolerable nuisance of swarms of aggressive and venomous insects, which infest the entire river region during the long summer season. Safety from the attacks of the numerous roaming tribes which ranged the country in every direction before it was definitely settled and organized, was also not among the last considerations. So, what nature had refused, the cunning and labor of man had to supply. Artificial hills or platforms were constructed, of enormous size and great height--from thirty to fifty, even sixty feet, and on their flat summits the buildings were raised. These platforms sometimes supported only one palace, sometimes, as in the case of the immense mounds of Koyunjik and Nimrud in Assyria, their surface had room for several, built by successive kings. Of course such huge piles could not be entirely executed in solid masonry, even of crude bricks. These were generally mixed with earth and rubbish of all kinds, in more or less regular, alternate layers, the bricks being laid in clay. But the outward facing was in all cases of baked brick. The platform of the principal mound which marks the place of ancient UR, (now called MUGHEIR),[H] is faced with a wall ten feet thick, of red kiln-dried bricks, cemented with bitumen. In Assyria, where stone was not scarce, the sides of the platform were even more frequently "protected by massive stone-masonry, carried perpendicularly from the natural ground to a height somewhat exceeding that of the platform, and either made plain at the top, or else crowned into stone battlements cut into gradines."[I] [Illustration: 10.--MOUND OF NIMRUD. (Hommel.)]