The project was entrusted to the priest-architect Guarino Guarini who, after leaving Paris in 1666, stopped in Turin in 1667 and took over the work on the chapel in 1668. He adopted the round-shaped project previously developed by Bernardino Quadri, which had been completed up to the first level. Guarini modified the previously completed structure, reinforcing the walls which had caused problems for their thinness and structural capabilities. He decorated the Corinthian-inspired pilasters with images and symbols linked to the Passion of Jesus (including olive branches, crowns of thorns, passion flower and the Titulus crucis). Furthermore, he reshaped the right staircase. He reduced the spandrels from four to three, creating a triangle with three vestibules at the vertices (one at the entrance from the palace and the other two at the staircases towards the cathedral). As for the portion of the chapel not yet undertaken, Guarini completely revolutionized the project. In particular, Guarini redesigned the dome, in order to lighten it and give it an upward thrust. The original project included a hemispherical dome supported by piers. Guarini instead placed ornamental pediments and shell-shaped volutes. Above this, he placed large weight-bearing arches which redistribute the weight. Inside each of the three lunettes and three spandrels formed by the three arches he placed large circular windows. These arches, in addition to concealing the buttresses, also made it possible to tighten the dimensions of the drum and the base of the dome. Above the arches, on the next level, he placed a ring walkway surmounted by six large arched windows alternating with six tabernacle-like niches. To build the top of the dome, he created a series of six levels, each made up of six small arches arranged in a hexagonal plan. The size of each level shrinks with height and the hexagon plan of each floor is slanted in respect to the one below, so that the arches place their bases on the top of the arches below. The dome ends with a roof lantern formed by a twelve-pointed star surrounded by twelve ovoid windows. Inside the star, a small circular cap is decorated with the dove of the Holy Spirit from which luminous rays emerge. The dome ends with a pinnacle decorated with windows (real at the base and fake at the top) with a golden globe surmounted by a cross on top. In 1680, Giacomo Cortella frescoed the dome.
