The Basilica of Santa Maria degli Angeli e dei Martiri is completely integrated into what were once the monumental Baths of Diocletian, built between 298 and 306 AD. In 1561, thanks to the strenuous efforts of the Sicilian priest Antonio del Duca, a devotee of the cult of angels, he obtained permission from Pope Pius IV to build a basilica dedicated to angels and Christian martyrs. This location was chosen because of a vision the priest himself had: a beam of white light emanating from the baths and rising to the sky, containing seven martyrs killed during the construction of the baths themselves.

The baths remained partially in use until the 6th century, then they were abandoned. Despite the looting and plundering they suffered, the central core of the baths, consisting of the tepidarium and frigidarium, was preserved into the Middle Ages. In 1561, Pope Pius IV commissioned The elderly Michelangelo, who was working on St. Peter's Basilica at the time, designed the church. Michelangelo's project restored the existing structure of the baths, integrating the area that constituted the frigidarium. After Michelangelo's death, the work was continued by Jacopo Del Duca, nephew of the priest Antonio Del Duca.

The basilica underwent several interventions starting in 1700: the orientation of the building was changed, the original entrance on the eastern side was closed and moved to the southern side, where it remains today. For the Jubilee of 1750, Luigi Vanvitelli intervened, giving the interior its current appearance, attempting to recreate an architectural uniformity: eight masonry columns were added, imitating those already present in the transept, and four chapels were opened. Vanvitelli built An imposing classical-style façade, however, was demolished in 1911 to restore the original calidarium niche with two arched entrances.

When entering the church, the first room you encounter is the circular vestibule. When it was still part of the baths, this was probably a nymphaeum leading to the calidarium. The transept, on the other hand, includes the frigidarium and is covered by three vaults supported by eight monolithic columns: those in the left transept are made of pink Egyptian granite, those on the right are made of masonry.

The basilica is decorated with frescoes and colored marble. On the splendid floor, you can admire the Sundial or Clementine Line, inaugurated in October 1702 by Pope Clement XI and designed by astronomers Francesco Bianchini and Giacomo Filippo Maraldi: the work is a 44-meter-long bronze line inserted into a marble band that crosses the church. On the sides of this line are the figures of the constellations and the numbers indicating the stars; at the ends are depicted the zodiac signs of Cancer and Capricorn. The purpose was to verify the date of Easter with extreme precision and establish noon with precision. In fact, on the wall of the basilica, there is a small hole in the center of the papal coat of arms that allows a ray of sunlight to pass through, marking solar noon on a different point of the line depending on the date of observation, moving from one end to the other. on the day of the summer solstice and the other on the day of the winter solstice.

The square in front of the basilica, Piazza della Repubblica, was formerly called Piazza Esedra because of the large exedra of the Roman baths, whose semicircular perimeter is traced by elegant porticoed buildings dating back to 1887-1898, the work of the architect Gaetano Koch.

The Fountain of the Naiads in the center of the square is the work of Palermo-born Mario Rutelli, created in 1901. The naiads represented are the Nymph of the Lakes, recognizable by the swan, the Nymph of the Rivers, reclining on a river serpent, the Nymph of the Oceans, riding a seahorse, and the Nymph of the Underground Waters, perched on a lizard. In 1912, the Glaucus group was added, again by Rutelli, symbolizing man's dominion over the force of nature.