The temple of the Pantheon in Rome was built on the area of the "Campo Marzio", where, according to an antique tradition, the founder of city, Romulus, would have reached the sky.
The first part of the structure, built in 27 before Christ on the order of the Consul Marco Agrippa, is made of a square temple in Greek style consecrated to the cult of all the divinities who were adored in the various regions of the empire.
After earthquakes and fires, only ninety years later, Adriano thought it was necessary to proceed to a deep restoration. Between 118 and 128 after Christ, in fact, the building sees various modifications in the plan with the addition of the pronaos, and the building of the largest cupola in beton never realized.
Even though the remaining structure of the Pantheon can be considered very different from the previous one, the imperator Adriano wanted that, on the fronton of the portico, was placed an inscription remembering the first commissioner: "M.Agrippa L:F: Cos Tertium Fecit" – "It was built by Marco Agrippa, son of Lucio, consul for the third time".
In 608 after Christ, Foca, imperator of Bisanzio, gives the temple to Pope Bonifacio IV who, after he has consecrated it to the Christian cult, calls it "Sancta Maria ad Martyres"; a few years later, in 663 after Christ, Costante II, new imperator of the oriental empire, gives the order to dislodge and take to Constantinople the tiles in bronze in golden bronze which were recovering the pronaos, but these are stolen by the Arabs during the transportation via sea. Between 1623 and 1644 after Christ, on the order of Pope Urbano VIII Barberini, the bronze decorations of the portico are in part allocated to the construction of the Baldachin of Bernini in San Pietro and in part to the fusion of some cannons for the fortress of Castel Sant'Angelo.
This episode, together with various spoliations of construction materials that in these years were perpetrated on monuments of the imperial Rome, contributed to the birth, among people, of the following dictum: "Quod non fecerunt Barbari, fecerunt Barberini" – "What the barbarians did not do, the Barberini did it". In the same years, under the will of the Pontificate to increase the clerical character of the structure, Bernini projects and manages the works for the construction of two belfries on the sides of the tympanum of the pronaos which, right away invisible to the citizens, tend to be called "the ears of ass", and which will be eliminated at the end of the 19th century. The Pantheon cloaks behind the portico with 16 columns of granite, more or less 14 metres high and with a circumference of more than 4, a circular environment illuminated only by the hole of 9 metres of diameter of the "oculus" located on the top of the cupola.
To reach the 43 metres of highness and at the same time of diameter of the cupola, the Roman architects were constrained to progressively lighten the charges, using materials always lighter and densities always slighter as the construction was proceeding towards the top. While the basement walls in travertine are roughly 6.5 metres large, going up for the 5 remaining concentric sectors of which the Pantheon is made of, the thickness decreases up to 1.5metre, and we see rings in admixture of travertine and volcanic tuff, in tuff and bricks, in bricks, in bricks and hearthstone, and, at the top of the structure, in vases of empty terracotta and hearthstone. Inside, the modifications in comparison to the prospects of the imperial era are essentially due to the sacred furniture of the church and to the presence of funerary monuments of Baldassarre Peruzzi, Taddeo Zuccari, Raffaello Sanzio and the royals of Italy.