[16] For this reason, Hypatos John II of Gaeta (933-963) planned a series of works (probably started in 954)[17] to give the martyr's relics a suitable location, purchasing a house contiguous to the cathedral to enlarge the church and build the new episcope.

[12] In 1003, Bishop Bernard, son of Marinus II, initiated the construction of the baptistery of St. John by purchasing a house located near the cathedral and thus beginning the “process of monumentalization of the northern front of the episcopal complex,” probably consisting of a series of staircases and loggias;[19] the designer of the baptistery was possibly Stefano, mentioned in a document of Bishop Leo IV in 1052; the same document describes the monumental access to the complex from the portico side, which took place with a double staircase: the one that led into the church and the one that, instead, through the underground vault reached the episcope.

[40] With the bull In Sublimi of Dec. 31, 1848, Pope Pius IX, exiled in Gaeta, elevated the diocese to the rank of archdiocese;[42] he himself visited and officiated several times inside the cathedral during his stay in the city (1848-1849) and again in April 1850 on his return journey from Portici to Rome, and donated some liturgical furnishings to it that are currently on display at the diocesan museum.

On January 22, 1903, on the occasion of the sixteenth centenary of the death of St. Erasmus,[50] construction began on a new façade in the neo-Gothic style, designed by Pietro Giannattasio and with the advice of Canon Filippo Pimpinella, in the same manner as Guarinelli had proposed.

[55] On the occasion of Pope John Paul II's visit to Gaeta (June 25, 1989),[56] it was decided to endow the cathedral with a new cathedra, and Erasmo Vaudo was given the task; the seat was made of marble from Coreno Ausonio, reusing some medieval sculptural relics belonging to the cathedral (such as two stylophoric lions placed at the sides of the seat, a marble fragment with cosmatesque quincunx lacking the mosaic decoration with the function of a backrest, and above it the eagle already present on the 18th-century facade),[57] and was inaugurated by the pope himself during the meeting with the diocesan clergy that took place inside the building.

[59] Starting in 2008, the cathedral underwent a major renovation to restore and radically alter its interior appearance: some of the ancient columns placed in the pillars between the central nave and the side aisles were brought to light; a new flooring in polychrome marbles with elements in the Neo-Cosmatesque style was made; and the entire layout of the presbytery was reorganized using elements from various periods and characteristics, including some bas-relief carved tiles, previously in the former church of Santa Lucia,[60] long considered to belong to the parapet of the cathedral's ancient pulpit and actually part of the plutei of that church.

At the same time, the columns that originally supported the chancel were placed in the atrium, with silvered bronze processional statues of the patron saints above them, on the left St. Erasmus and on the right St. Marcianus, made by Erasmo Vaudo in 1984-1985 and already located inside the cathedral, first in the apse and then in the second side chapel on the right.

[70] The basement is internally hollow: in the main façade of the bell tower, which faces the gulf, there is a large ogival arch supported by two Tuscan columns; this gives access to a room covered with a rib vault and entirely occupied by a staircase ending in a doorway, which constitutes a secondary entrance to the cathedral.

The top consists of an octagonal dome flanked by four smaller circular turrets; the entire complex is decorated with polychrome enamels[73] and numerous painted ceramic basins, currently 20th-century copies of the four types of original 13th-15th century ornamentation.

[69] To the right of the bell tower, along the outer wall of the apse, there are some windows in neo-Romanesque style: mistakenly considered as the only remains of a possible baptistery present in that area and dedicated to St. John the Baptist, they date back to the 20th century and provide light to a room originally conceived with a staircase function.

[67] The counter façade of the nave is characterized by the presence of the main entrance portal, surmounted by the altarpiece, formerly on the back wall of the apse, Martyrdom of St. Erasmus by Carlo Saraceni[75] (made in about 1610-1612 probably on commission from the Bishop of Gaeta Pedro de Oña, O. de M.);[76] deeply damaged by the bombing of 1943 and subsequently subjected to a radical restoration, the work is one of the author's few large paintings, and for this reason it is characterized by a strong stylistic discontinuity between the lower part (very realistic and clearly inspired by Caravaggio) and the upper part (of academic derivation).

[78] On either side of the portal are walled two ancient marble balustrades decorated in bas-relief (of which the one on the entrance to the right, with ornate intertwined wicker circles combined with crossed diagonals, comes from the church of San Domenico in Gaeta)[79] and four epigraphs commemorating the visit of Pope John Paul II (June 25, 1989), the visits of other pontiffs to the city, the bishop and humanist Francesco Patrizi (in 1775) and the presence of Pius IX in Gaeta as well as the elevation by his will of the diocese to an archdiocese; the latter was commissioned to be sculpted by Archbishop Filippo Cammarota in 1857 and was part of a larger monument placed on the wall to the right of the doorway of the sacristy of the hebdomadaries.

The floor of the hall and chapels, made of sawn white and yellow marble in imitation of the post-war one in the apse, dates from the 2008-2014 restorations, as does the neo-Cosmatesque-style fascia in the nave, which features a modern design with five spiral disks regularly arranged along the vertical axis (analogous to that of the late 12th-century church of San Pietro in Vinculis in Pisa)[82] and a quincunx at the foot of the chancel, at the base of which is carved the signature of Franco Vitelli, who created the work.

[85] Each of the two side aisles ends with a double flight of steps: the upward one leads to the presbytery, the downward one, on the other hand, to the crypt; in the fourth bay, raised above the others, an altar in polychrome marble finds its place: the one on the left, by Domenico Antonio Vaccaro, houses the polychrome wooden statue of the Immaculate Virgin (early 20th century); the one on the right, dating from 1828 and deprived of the mensa, the coeval wooden statue of St. Joseph[86] and Sebastiano Conca's oil on copper painting Jesus Crucified between the Madonna, St. John the Evangelist and St. Mary Magdalene (1764), located within a marble frame above the tabletop, which recalls Luca Giordano's Jesus Crucified with the Three Marys and John the Evangelist (c. 1690-1692)[87] placed on the left side altar of the nave of the sanctuary of the Santissima Annunziata in Gaeta.

[89] In the seventh aisle there are additional fragments as well, embedded in a movable platform already in use for the present ambon: the two larger ones are the one that constitutes the floor of the upper step, decorated with "an original boxed layout and T-band,"[90] and the one on the left side that features a cross inscribed in a clypeus; the three smaller ones consist of the aforementioned pavement slab with a zig-zag motif, one with opposing triangles alternating with diagonally placed squares (both on the right side), a third with alternating squares and rhombi with triangles (modern replica, on the first step), and a fourth analogous to the mosaic cornices of the balustrades that make up the chancel elevation.

Pictorial traces (partly reduced to sinopia) can also be found on the remains of the Gothic vaults still visible in the attic of the first bay of the present right aisle, probably dating from the early 15th century and depicting in addition to a star motif the evangelists Mark and Matthew.

[85] On the two side walls are paintings: on the left, a Flagellation of Christ[85] by an unknown 17th-century author of the Neapolitan school, from the collections of the National Museum of Capodimonte and granted in 1938-39 to the church of San Domenico in Gaeta; on the right, Madonna of the Rosary by Sebastiano Conca (18th century).

The third chapel on the right has no altar: it is deeper than the others to the extent that it occupies two aisles of the 13th-century building in width, and its primary function is to connect the cathedral's rear entrance, consisting of the portal that is at the top of the staircase of the base of the bell tower, to the hall.

[58] During the 2008-2014 restorations, it was turned into a Bourbon shrine, which was inaugurated on November 29, 2014,[110] bringing into the church some sepulchral monuments already kept in some neoclassical rooms located below the hall, which were accessed through a flight of stairs in the left bay of the atrium.

The cathedra is surmounted by a 15th-century bas-relief angel by Domenico Gagini, while on the wall above the seat are a fragment of an arched slab with a vine leaf motif and traces of polychromy,[142] and a detached fresco from the former church of Santa Lucia depicting the Annunciation.

Following the construction of the neo-Gothic elevation, it was placed in the right bay of the atrium, resting on the currently dismembered group of four stylophoric lions in the pronaos; it was moved back inside the cathedral in 1920, under the second archway between the nave and the right aisle.

[145] The work can be attributed to workers linked to the workshop of Pellegrino da Sessa and, due to the absence of decorative mosaic elements, is analogous to that in the basilica of St. Paul Outside the Walls in Rome (made by Pietro Vassalletto and Nicolò D'Angelo around 1170) and to that in the Palatine Chapel in Palermo (dating from around 1260-1280).

[108] The back wall of the apse, behind the lower portion of which rests the ancient Baroque high altar, is divided into three bays by pilasters: in the central one, until 1976, found its place the surviving part of the Lepanto Standard, the work of Girolamo Siciolante da Sermoneta, donated to the cathedral by Don John of Austria; the painting depicts Jesus crucified between Saints Peter and Paul and was kept first in the picture gallery of the Historic Cultural Center of Gaeta,[153] then in the Diocesan Museum, where it is currently located.

[156] With the 2008-2014 restorations, a panel painting, formerly in the capitular sacristy, depicting Madonna and Child with St. Michael the Archangel surrounded by a court of six angels, from the church of Sant'Angelo in Planciano, was placed in the center; dating from the 1560s, it has been mistakenly attributed to several artists, such as Bronzino, Andrea Sabbatini or Fabrizio Santafede,[157] and later to the Sienese Marco dal Pino;[86] its author, however, has been identified as Giovanni Filippo Criscuolo due to numerous similarities with other works by the artist: the Madonna and Child, which occupies the upper part of the painting, bears a remarkable resemblance to the Raphaelesque Madonna of Grace present in the church of Santa Maria della Mercede a Montecalvario in Naples, while in the lower register the figure of the archangel Michael, on whose sides six angels are symmetrically arranged, recalls the similar one present in the center, below, of the Last Judgment attributed to Criscuolo[158] and currently at the Bob Jones University campus in Greenville, South Carolina.

Due to the bombing of 1943, the central section of the antependium was almost completely lost, which was no longer the original one, but a 1786 remake in late Baroque style to allow the veneration, through an oculus, of the remains of St. Albina; one in simpler forms was then made, with a red and white marble urn in relief on a green background.

The two side fields of the antependium, on the other hand, date back to the 17th century and echo the style of the rest of the altar, with rich inlaid decoration in polychrome marble and mother-of-pearl depicting plant elements, floral cups and scrolls, which also continues on the three steps of the riser.

Above the tabernacle are two corbels: on the lower one was placed the silver statue of St. Erasmus on the occasion of the patron's feast day[163] (dating back to the 14th century and several times modified and enriched, stolen in 1981);[164][165] on the upper one, considerably elevated, is the valuable monumental Crucifix in gilded and silver-plated wood, coeval with the altar and also probably the work of Dionisio Lazzari; the sculpture presents Christ hanging from an elaborate cross resting on a high base with a globe and flanked by two cherubs.

[166] On the lower volutes placed at either end of the altar to support the two upper steps of the riser (each of which is surmounted by a similar element terminating in an angel's head), are the inlaid coats of arms of bishops Lorenzo Mayers Caramuel, O. de M. (right) and Martino Ibáñez y Villanueva (left).

[168] The cathedral's first pipe organ was probably built in the 16th century; identifiable as the front part of the latter's case is the frame of the triptych Coronation of the Virgin by Giovanni da Gaeta, formerly in the former church of Santa Lucia and now in the Diocesan Museum; it is characterized by the presence, on the top, of the gilded sculptures of Saints Erasmus, Michael the Archangel and Marcian.

The phonic material is integrally enclosed within a wooden case soberly decorated with carvings and moldings, with a display consisting of tripartite diapason reeds within a serlian opening, closed by small doors; the console is windowed.