The exterior is covered with white marble that abounds in the area, with a salient facade, from which the internal profile with the three naves can be guessed.
On each of the three portals there are sculpted lunettes showing as many scenes from the Passion and Resurrection of Christ, generically attributed to the Pisan school. In the right transept another door opens onto Via Garibaldi, also surmounted by a sculpted lunette depicting St. John the Baptist, a fourteenth-century work by Bonuccio Pardini.
The coats of arms on the facade recall the Genoese and Florentine domination of the city, in addition to the large coat of arms of Pope Leo X. The fine marble rose window is attributed to Riccomanno Riccomanni (14th century).
The bell tower that flanks the collegiate church is about 36 meters high, with a square base plan with a side of 8 meters, and has a simple exposed brick surface, even if the original project envisaged a marble cladding, which was never completed. The interior is provided with a particular helicoidal staircase.
The bell tower hosts one of the most prestigious bell concerts in all of Versilia. Manufactured by the historic Lorenzo Lera Foundry of Lammari (Lucca) in 1887, each bell has particular dedications or denominations. The big bell is dedicated to the Madonna del Sole, the mezzanine to San Martino (owner of the Cathedral), the mezzanine is dedicated to San Costanzo of Perugia and the small one is called Ave Maria (or “Ave Marietta” in the common people). Very curious is the fact that, when the bells ring out, the bell tower begins to sway considerably due to its entirely brick structure. This elasticity also gives the tower excellent resistance to seismic shocks.
The bell tower, commissioned by the collegiate church of San Martino was completed around 1520 and is attributed to the Florentine architect Donato Benti, but some sources claim that Michelangelo Buonarroti also contributed to its construction, above all in the construction of the complex and characteristic helicoidal staircase.
The plan of the church is a classic Latin cross with three naves and a transept.
The preserved works date back to various eras, but the period that has most influenced the current appearance is that linked to the Grand Duchess Christina of Lorraine who, in 1627, commissioned Florentine artists to renovate the church: the side altars were created, which were decorated by large devotional altarpieces (there are works by Matteo Rosselli, Francesco Curradi, Jacopo Vignali, Pietro Dandini, Bastiano Bitozzi, Jacopo Chiavistelli and Alessandro Cominotti), while the sculptural apparatuses were curated by Giovan Battista Stagi and Ferdinando Tacca.
The marble pulpit, a valuable work by Donato Benti and Lorenzo Stagi (1508) and the two stoups by Stagio Stagi date back to the previous century.
The walls and ceilings were painted with various subjects in color or chiaroscuro by the Milanese painter Luigi Ademollo between 1823 and 1825. The dome was originally octagonal (1453), but was rebuilt with a circular plan in 1820.
In the transept chapel to the right of the main altar is the venerated image of the so-called Madonna del Sole, a painting on wood with a Madonna and Child among saints by an anonymous late Gothic painter, dated 1424 and which is only exhibited during special celebrations.
In 1784 the pipe organ of the oratory of the confraternity of the Santissima Trinità of Pistoia was transferred to the collegiate church, for which it had been built by Domenico Francesco Cacioli together with Antonio and Filippo Tronci between 1748 and 1749; this instrument was in turn sold in 1833 to the parish church of San Michele in Farnocchia and replaced by another specially built by Giosuè Agati; the latter had 48 registers on two manuals and pedal and was enclosed within a richly decorated neo-Gothic style case. It was restored by Odoardo Landucci in 1852 and rebuilt by the Agati-Tronci company at the end of the 19th century.
The nineteenth-century organ was demolished and replaced in 2017-2018 by a new one built by the Chichi company; the latter is electronically transmitted and has 54 registers (on three manuals and pedal) for a total of 2663 pipes. The instrument is divided into three bodies: the main one is on the choir loft in the counter-façade while the other two are located behind the high altar and in the space below the right transept.