Side view of the Papal Basilica of St. Paul outside the Walls (Italian: Basilica Papale di San Paolo fuori le Mura), commonly known as St. Paul's outside the Walls, is one of Rome's four ancient, Papal, major basilicas, along with the Basilicas of St. John in the Lateran, St. Peter's, and St. Mary Major.
The Basilica was founded by the Roman Emperor Constantine I over the burial place of St. Paul, where it was said that, after the Apostle's execution, his followers erected a memorial, called a cella memoriae. This first edifice was expanded under Valentinian I in the 370s.
In 386, Emperor Theodosius I began erecting a much larger and more beautiful basilica with a nave and four aisles with a transept; the work including the mosaics was not completed until Leo I's pontificate (440–461). In the 5th century it was larger than the Old St. Peter's Basilica.
Under Pope St. Gregory the Great (590-604) the Basilica was extensively modified again. As it lay outside the Aurelian Walls, the Basilica was damaged in the 9th century during a Saracen raid. Consequently, Pope John VIII (872-82) fortified the Basilica.
On 15 July 1823 a fire resulted in the almost total destruction of the basilica which, alone of all the churches of Rome, had preserved its primitive character for 1435 years. It was re-opened in 1840, and reconsecrated 1855. Completing the works of reconstruction many countries made their contributions. The Viceroy of Egypt sent pillars of alabaster, the Emperor of Russia the precious malachite and lapis lazuli of the tabernacle. The work on the principal façade, looking toward the Tiber, was completed by the Italian Government, which declared the church a national monument.
more on https://en.wikipedia.org/wiki/Basilica_of_Saint_Paul_Outside_the_Walls