The monumental staircase of 136 steps, commissioned by Cardinal de Tencin, was inaugurated by Pope Benedict XIII on the occasion of the Jubilee of 1725; it was built (thanks to French funding from 1721-1725) to connect the Spanish Bourbon embassy (to which the square owes its name) to the church of Trinità dei Monti.
It was designed by both Alessandro Specchi and Francesco De Sanctis after generations of long and heated discussions on how the steep slope on the Pincio side should be urbanized to connect it to the church. The final solution chosen was that of De Sanctis: a large staircase decorated with numerous garden terraces, which in spring and summer is beautifully decorated with many flowers. (wiki)