St. Martin's is located in a cavity of the west wall above the Porta Aurea of Diocletian's Palace. That space, in the time of Diocletian, was a narrow corridor that was used as a guardhouse, with the windows on the south side for surveillance of approach to the main gate into the complex. These windows remain well preserved to the present day in their original form, while windows on the north side date from the city's defense against the Ottoman Turks in the 15th century. The space was converted into the church some time in the 6th century when the complex saw an influx of refugees from outlying communities. Similar churches exist/existed over the Silver Gate, Iron Gate, and the Bronze Gate. An inscription along the architrave of the entrance door: REX BENEDICTE DS, LAUDIS DECUS IMPERI VQANC CONSTITUIT SUB ONORE BEATI MARTINI UMILTE EC ATQ POTES PBTR DOMINICUS. SALVE PA...T TIBI P NRI PECTORIS ANTRA COLI.
Translated by archaeologist Frane Bulić, it reads: In identifying Dominic, Bulić dated the inscription to the ninth century and the region of Trpimir I. It seems the chaplain Dominic renovated the former guardhouse into the first Christian church in the city dedicated to the Virgin Mary, Saint Gregory I and St. Martin of Tours. On 4 March 852 Trpimir issued a charter in Biaći in the Latin language, confirming Mislav's donations to the Archbishopric in Split. The charter is preserved in a copy from 1568. In this document, Trpimir named himself "by the mercy of God, Duke of the Croats" and his realm as the "Realm of the Croats". The term regnum was used by other rulers of that time as a sign of their independence and did not necessarily mean a kingdom. The charter documents Trpimir's decision to build a church and the first Benedictine monastery in Rižinice, thus bringing the Benedictins into Croatia. On the architrave, together with his own name, Trpimir also had the name of his other chaplain. The Main altar too dates from the 9th century. The church later provided certain amenities such as a settlement for Dominican sisters. Bulić, following his discoveries of 1890 and study of the palace, recommended its renovation. On that occasion, the new altar and nave were built. In 1929, during the enlargement of one of the windows, a tablet was discovered with an engraved inscription: "The unworthy servant, priest Dominic, chaplain of Duke Trpimir". Bulić believed that this might represent the gravestone of Dominic. Today it is kept in the Archaeological Museum. Above the church, a pre-Romanesque bell tower was erected, which was later demolished in the 19th century.
Architecture
The northern gate of the Palace had been conceived as the main entrance and so it had been elaborately decorated with statues of Emperor Diocletian and his co-regent Maximian, in the upper row of the niches, the a–nd with sculpture of an eagle, between the two. There were the statues of their two successors to the throne – CaesarsGalerius and Constantine – in the lower row of the niches. This conclusion can be made on account of the imperial iconography pattern, effective at the time of tetrarchy. On the top of the wall, there have been four pedestals preserved until the present day, that could have been used as a supporting base for statues, but it is unlikely to believe that the imperial figures would have been repeated twice on the same façade. So far there has not been a credible explanation given for this double usage. This could be concluded by the presence of patron saints, to whom the other gates of the Imperial Palace had been dedicated: St. Theodore, St Apollinaris and St. Julian. St. Martin, like St. Theodore, was venerated in the later Roman period, particularly in the West, during the rule of the Emperor Justinian. The altar screen divides the church into two parts. The screen is made of marble and covered in vines, grape vines and griffon. On the altar wall, the only one preserved in situ in Dalmatia, there is an inscription with the dedication to patronage the Virgin Mary, St. Gregory the Pope and Blessed Martin. The pre-Romanesque stage, probably built in the 9th century, belongs to the barrel vault, with an altar in the apse with a carved cross of early Christian denominations and a small trance, set in the middle of large, buried antique openings on the southern wall. The later pre-Romanesque stage of the 11th century belongs to the altarpiece and the bell tower, which was later destroyed.