After the great cathedrals, this is the most impressive monument built in Sicily in the middle Ages: here you can admire the syncretism of the Byzantine, Arab and Norman arts and religions. It dates back to the 12th century, as it can be read from Roger II's donation diploma in favour of the monastery belonging to the Abbot Gerasimo. The diploma shows that in 1116, during a stop at Sant'Alessio Siculo, the king came to meet the Abbot Gerasimo who asked to grant him a license and means to rebuild the monastery, previously Byzantine, and located along the river Agrò. He granted the necessary sum, territories of the Valle d'Agro and census rights. The planimetric scheme according to some authors served as a model for the Palatine Chapel in Palermo. The entrance with a portico between two towers is typical of the Norman architecture. The Greek epigraph engraved in the lintel of the portal reads: “This temple of the Apostles Peter and Paul was built by Teostericto a catechumen tauromenita at his own expense. May God remember him in the Year 6680.
It is the only Norman synchronous inscription with the construction date and the manufacturer's name.
As many scholars claim, the intervention of Gerardo il Franco in 1172 was a restoration after the terrible earthquake of 1169 and was limited to the main portal and the south gate. The restoration, including Byzantine, Arab and Norman, was carried out by an architect demiurge. The plan is divided into three naves with apses, separated by columns and pointed arches. The main dome rests on four granite columns, the smaller one grafting at the intersection between the transept and the presbytery. Domes held up and pushed upwards by splendid hanging alveolar niches of Islamic origins, the muqarnas, arranged in staggered turns, extending more and more forward, which transform the quadrangular plant into a circle in order to insert a tambour and a calotte. The external decorative elements are Byzantine pilaster strips connected to the masonry, which intertwine in blind arches of Islamic derivation. The colour effects are obtained thanks to the use of bricks, sandstone, stones and Taormina stone and the tactile effects of herringbone, knife, and plate or saw-tooth wall texture. The polychrome decoration is typical of the Norman architecture of the province of Messina and some authors attribute it to the various religious orders: that is, to the Basilians who exercised the Greek rite, a type that can also be found in the churches of Santa Maria di Mili, San Filippo di Fragalà and Santi Pietro and Paolo di Itàla and constitutes an evident persistence of the "Opus Spicatum" of the Roman buildings of decadence. The defense against the frequent Arab medieval incursions was given by the fortified aspect of "ecclesia munita" with battlements and the orientation towards the azimuth of the liturgical dawn of the martyrdom of Teodoro d'Amasea, patron saint of the Byzantine army. The monastery was the driving force of Christianity and culture. The monks raised hymns in the melodious Greek idiom and at the same time in the scriptorium the ancient codes were studied and illuminated. The Sicilian copyists, and probably also those of the monastery of Saints Peter and Paul of Agrò, were famous for the elegance of the imitations of the great Byzantine colored initials in carmine. The Basilians with the Cross and the plow, with prayer and work, taught the peasants to plow the land and fertilize it. Numerous water mills, paths, viaducts and waterways rose throughout the valley. The monastic community moved definitively in 1794 to Messina "because of the unhealthy air, contaminated by the pestiferous water of the linen", in a building located in Via I Settembre. The monastery and the Basilica became the property of some private individuals, including the Crisafulli family of Casalvecchio, who had the body of the joint anti-Bourbon hero buried there. In 1904 the temple was purchased by the State Property Office and in 1909 declared a national monument. With the advent of humanism, a hunt was launched for rare books and ancient manuscripts. The Spanish scholar Gonzalo Perez, acquired 35 Greek manuscripts owned by the abbeys of Itàla and Agrò. Of these, nine came from this monastery and were later donated by Philip II to the Madrid Escorial Library. The presence of these texts was documented for the first time by Cardinal Mercati in 1930. The library, lost in the Spanish conquest, was digitally refounded in 2016 thanks to the synergy between Lions Club Letojanni-Valle d'Agrò, Archeoclub Ionian Area Messina, municipality of Casalvecchio Siculo and citizens: the texts were purchased in digital format and placed in the totem placed in one of the apses.
