But by far the most formidable military invention of the warlike Assyrians was the battering-ram; the ram was brought to bear upon the wall of the besieged city by a movable tower, in the shelter of which the ram could be effectively and safely worked, the tower and the battering-ram thus forming together a most potent factor in both offensive and defensive operations. These movable towers were by no means uniform, but varied both in size and height, sometimes they were surmounted by towers (cf. Fig. 111 (A)) from which the attacking forces could shower their arrows upon the beleaguered army with impunity, at other times they were quite low and shaped liked a torpedo, the larger ones resting on six wheels (cf. Fig. 44), and the smaller on four (Q). The ram itself also varied—sometimes it was set at an angle slanting upwards (A), its projecting extremity being at the same time heavier and thicker than the shaft, but more usually the ram was fixed horizontally and pointed like a spear (B), the tower sometimes being armed with two of these rams (C). The most noticeable of the shields here represented are the large shields, from behind the shelter of which the bowman could aim and shoot at his ease, the shield of course being held in position by a shield-bearer (cf. (D), (E), (F)). These large shields were generally upright (F), but were often curved at the top to protect the head of the archer from the missiles of the enemy (D), while sometimes the whole shield was curved (E). But the lancers required no such protection, a small hand-shield which they could carry themselves being the only type of defence which would not completely nullify their usefulness in the field. These shields varied in shape and size; they were generally round (cf. (G)-(K)), but sometimes curved and oblong (L), while at other times they were concave in the body, oval at the top, straight at the bottom, and decorated with a boss in the centre and an engraved design round the edge (cf. (N)). Another type of shield was shaped somewhat like a lozenge (O), but they all alike have their handles in the centre. They were often most elaborately engraved, the designs being formed by an arrangement of straight lines ((G) and (P)), geometrical figures ((H) and (L)), or circles of rosettes ((I) and (J)). One of the shields illustrated here differs from the rest in having its outer face notched like the edge of a saw, and must have served offensive as well as defensive purposes (cf. (M)). [Illustration: FIG. 112.]