The area on which the church of San Giovanni Battista stands, as well as the territory of Assemini, has been inhabited since prehistoric times. Archaeological traces testify to the Phoenician-Punic and Roman phases. Epigraphic sources and parchment sources testify to its importance at the judicial age, when it was among the privileged residences of the rulers of Cagliari.
The church of San Giovanni Battista di Assemini is a unique monument of paramount importance in the field of Byzantine architecture not only in Sardinia, but also in the entire Mediterranean basin. The importance of the architectural forms is combined with that of the epigraphic testimonies in the Greek language kept inside, which maintain the memory of the oldest judge or king of Cagliari.
The building probably dates back to the tenth century. His first documentary mention dates back to 1108 and concerns the donation to the Opera of the cathedral of San Lorenzo in Genoa by the Cagliari judge Mariano-Torcotorio I de Lacon-Gunale.
The plan has a cross carved in a square of 10 m on each side, with an E-shaped apse. At the intersection of the barrel-shaped arms, four sturdy L-shaped pillars hold the dome inside a tiburium. The façade, about 5 m high, is dominated by a small sailing bell tower. The walls are made of limestone, squared at the corners and only outlined in many sections of the perimeter walls, while the vaults are made with greater accuracy.
On the sides of the arms of the cross there are four rooms measuring 3 m on each side, on the origin of which two hypotheses are given. The first considers them to have been built later: therefore, the church would have been born on a free cross. The second goes back to the cross plan inscribed with the original plant. Even on the chronology of the church there is no satisfactory agreement among scholars.
In the church there is a group of sculptural fragments that includes the two inscriptions with the names of Torcotorio Arconte, his wife Getite and Nispella, wife of a different Torcotorio, attested between the mid-tenth century and the middle of the eleventh century as the names of the highest authorities in Sardinia at the time of transition between Byzantine and judicial periods.

History of studies
The church was mentioned as early as the 19th century by Giovanni Spano. In 1953 Raffaello Delogu returned it as a cross engraved from the beginning, dating it to the 10th-11th century. In 1962 Bruno Virdis, on the other hand, resumed Francesco Giarrizzo's hypothesis, that it was originally a free-cross system, later expanded with the addition of corner chambers. The Middle Byzantine epigraphic marbles kept in the church have been extensively analyzed by Roberto Coroneo.

Bibliography
G. Spano, “Christian Antiquities of Assemini”, in Sardinian Archaeological Bulletin, VII, 1861, pp. 133-139;
A. Taramelli, “Assemini - Byzantine Inscriptions of the Church of St. John and of the Parish Church of St. Peter”, in News of the Excavations of Antiquity, 1906, pp. 123-125;
A. Taramelli, “Assemini - Byzantine decorative fragments recovered in the church of St. John the Baptist”, in News of the Excavations of Antiquity, 1919, pp. 161-168; F. Giarrizzo,
The church of San Giovanni di Assemini, Rome, 1920;
R. Delogu, The Architecture of the Middle Ages in Sardinia, Rome, The State Library, 1953, pp. 30-34;
B. Virdis, “Reliefs of three Sardinian churches. Saint John of Assemini, Saint Anthony the Abbot and Saint Lawrence of Cagliari”, in Palladio, XII, 1962, pp. 80-89; R. Coroneo-M. Coppola, Byzantine Cruciform Churches of Sardinia, Cagliari, 1999, pp. 23-25;
R. Coroneo, Medium-Byzantine Sculpture in Sardinia, Nuoro, Poliedro, 2000, pp. 62-65;
S. Mancosu, “Assemini and the Church of San Giovanni”, in City, Territory, Production and Commerce in Medieval Sardinia. Studies in honor of Letizia Pani Ermini offered by Sardinian students for her seventieth birthday, by R. Martorelli, Cagliari, AM&D, 2002, pp. 25-64; R. Coroneo-R.
Serra, Pre-Romanesque and Romanesque Sardinia, series “Italian Artistic Heritage”, Milan, Jaca Book, 2004, pp. 71-73; R. Coroneo, Romanesque Churches of Sardinia. Tourist-cultural itineraries, Cagliari, AV, 2005, p. 96.