The second and third of the principal concentric walls contain arches corresponding to those in the outer wall. Corridors run between these concentric walls, and on the first and second floors of the outer ring, and the first floor of the inner ring, these circles afford a completely unobstructed passage all round. The other corridors are blocked up in parts by various staircases, leading to the upper rows of seats. Within the third principal concentric arcade the supports of the building take the form of massive walls, radiating from the centre of the ellipse, and divided by elliptical corridors into three ranges. Between these massive walls and in the corridors are the steps and passages leading to the lower seats of the amphitheatre. The actual seats which were of marble have been all pilfered for the benefit of the Roman palaces and churches of the feudal ages, but we can still make out with tolerable certainty the five principal divisions into which they were separated. The lowest of these, called the podium, was a platform raised twelve or fifteen feet above the arena, upon which were placed the chairs of the higher magistrates and dignitaries. This was protected by railings and nets full of spikes, and sometimes also by trenches, called euripi, and horizontal bars of wood or iron which turned freely round, and thus afforded no hold to the paws of a wild animal. Above the podium were four different orders of seats, divided by belts of upright masonry from each other. The first of these consisted of about twenty rows of seats, and was appropriated to the knights and tribunes, and other state officers. The upper row of this set was probably at a height of about ten feet above the top of the arches of the lowest story. The next ranges of seats between the second and third belt were appropriated to Roman citizens in general, and held the greatest number of spectators. The wall dividing these seats from the next set was very high, and contained, besides the vomitoria or entrance doors, a number of windows for the purpose of lighting the corridors and passages. A considerable part of this wall is still extant upon the side towards the Esquiline Hill. Above it ran the third set of seats, occupied by the lower classes of the people, and above this again, and separated from it by a very low wall without vomitoria, was the fourth group of seats, immediately under the windows of the uppermost story, and covered by a portico which ran round the whole top of the building.[71] The traces of this uppermost row of seats and of the colonnade which supported the portico may still be seen on the side towards the Esquiline Hill.