At the back of the great temple on the Quirinal – often thought to be Aurelian’s Temple of the Sun, sometimes Caracalla’s Temple of Serapis – a great staircase ran down the hill to the plain. Portions of the sides of this still remain; but the actual arrangement of this is unclear. The steps themselves vanished in the 14th century, so even 16th century drawings must be treated with care.
Yesterday I found a 16th century drawing of this temple and staircase by Sallustio Peruzzi (Click to enlarge).
It is entirely consistent with Palladio’s plan, which leaves the disposition of the staircase unclear.
However Palladio also gives a nonsense reconstruction of the stairs, which is not consistent with his own plan:
It has misled the authors of the recent Atlas of Ancient Rome, whose diagram shows a block:
As I remarked earlier, the slope of the hill is gentle, and this enormous construction is simply unnecessary, and unevidenced.
Peruzzi is almost certainly right, I believe. His view is also consistent with this drawing of the stair side-walls from the period.
So… unless any further evidence is forthcoming, that’s how I think it should be seen. Palladio’s block is just an architect’s fantasy, and should not be considered.