Cordonata is the access ramp to the Piazza del Campidoglio. It was designed by Michelangelo.
The idea is that even horsemen could ride into Piazza del Campidoglio without dismounting. The transition between the balustrade and the stairs is guarded by giant statues of Castor and Pollux holding their horses.
Michelangelo only lived long enough to see these stairs being built. Palazzo Nuovo was completed in 1654, and not open to the public until 1734.