Inside the second concentric ellipse of piers begins a set of radial walls which supported the seats. The slope of the seats was perfectly calculated for perfect visibility. The vaults of the lower levels were left open until the upper level piers were finished. This made possible the use of derricks to lift heavy blocks to the upper levels. The third-story piers have one course of blocks projecting, to provide a step to support the scaffolding required for building the wall on the fourth level. This wall is built of smaller blocks than those used on the lower levels, to facilitate lifting, and it is full of second-hand materials; column drums, for example, which may have come from the Golden House. The outer face of the fourth-level wall is equipped with 240 consoles, projecting brackets jutting out from the wall to support masts. Corresponding to each in the cornice above is a hole for the mast. The mast, Cozzo argues persuasively, was fitted with rope and pulley. The rope descended obliquely and was fastened to another below which ran elliptically at a convenient height above the podium of the arena. Awnings, fixed to these ropes, could be rolled up or down in strips as the sun’s position dictated. Awnings being made of canvas, this duty was assigned to detachments of sailors--the logical Roman administrative mind at work. When the skeleton was finished, the space between the piers in the radial walls was filled in, on the ground level with tufa, on the second level with lighter materials, brick and cement. Only then were the vaults completed. The stairs were ingeniously planned to give access from the ground direct to each level separately. This both emphasized distinctions (VIPs in the lowest tier, women at the top; compare the separate second-balcony stairs in modern theaters) and facilitated entrance and exit. Each outside entrance--there were originally eighty--bears a Roman numeral. This corresponded to a number on the admission ticket, and divided the 45,000 or 50,000 spectators into manageable groups. The arena proper was surrounded by a wall, high enough to protect the spectators from the beasts (VIPs not being regarded as expendable), but not so high as to block the view of the arena from the seats behind. Slots in the top of this wall are the postholes for a dismountable fence which supplied additional protection. Literary sources say it was of gilt metal surmounted by elephants’ tusks. In front of the fence ran a catwalk where archers were stationed to shoot beasts which got out of hand.