Archbasilica of St. John Lateran, Rome, decorated ceiling and Nave with the high altar.

The Papal Archbasilica of St. John in the Lateran (Italian: Arcibasilica Papale di San Giovanni in Laterano), commonly known as St. John Lateran Archbasilica, St. John Lateran Basilica, St. John Lateran, or just The Lateran Basilica, is the cathedral church of Rome and the official episcopal seat of the Bishop of Rome, the Roman Pontiff.
It is the oldest and ranks first among the five Papal Basilicas of the world and the four Major Basilicas of Rome (all of which are also Papal basilicas), being the oldest church in the West and having the Cathedra of the Bishop of Rome.
The Archbasilica is in the City of Rome, outside the boundaries of Vatican City which is approximately 4 km (2.5 mi) to the northwest, although the Archbasilica and its adjoining buildings enjoy extraterritorial status as one of the properties of the Holy See pursuant to the Lateran Treaty of 1929.

The Archbasilica stands over the remains of the Castra Nova equitum singularium, the "New Fort of the Roman imperial cavalry bodyguards". The fort was established by Septimius Severus in AD 193. Following the victory of Emperor Constantine I over Maxentius (for whom the Equites singulares augusti, the emperor's mounted bodyguards had fought) at the Battle of the Milvian Bridge, the guard was abolished and the fort demolished. Substantial remains of the fort lie directly beneath the nave.
The remainder of the site was occupied during the early Roman Empire by the palace of the gens Laterani. Sextius Lateranus was the first plebeian to attain the rank of consul, and the Laterani served as administrators for several emperors. One of the Laterani, Consul-designate Plautius Lateranus, became famous for being accused by Nero of conspiracy against the Emperor. The accusation resulted in the confiscation and redistribution of his properties.
The Lateran Palace fell into the hands of the Emperor when Constantine I married his second wife Fausta, sister of Maxentius. Known by that time as the "Domus Faustae" or "House of Fausta," the Lateran Palace was eventually given to the Bishop of Rome by Constantine I. The actual date of the donation is unknown, but scholars speculate that it was during the pontificate of Pope Miltiades, in time to host a synod of bishops in 313 that was convened to challenge the Donatist schism, declaring Donatism to be heresy. The palace basilica was converted and extended, becoming the residence of Pope St. Silvester I, eventually becoming the Cathedral of Rome, the seat of the Popes qua the Bishops of Rome.
The official dedication of the Archbasilica and the adjacent Lateran Palace was presided over by Pope Sylvester I in 324, declaring both to be a Domus Dei or "House of God." The Papal Cathedra was placed in its interior, making it the Cathedral of the Bishop of Rome. During the time the Papacy was seated in Avignon, France, the Lateran Palace and the Archbasilica began to deteriorate. Two destructive fires ravaged them in 1307 and 1361. After both fires the Pope in Avignon sent money to pay for their reconstruction and maintenance. Despite those expenditures the Archbasilica and the Lateran Palace lost their former splendor. There were several attempts at reconstruction of the Archbasilica before a definitive program of Pope Sixtus V (1521-1590). Some portions of the older buildings survive. Among them the pavement of mediaeval Cosmatesque work, and the statues of St. Peter and St. Paul, now in the cloister. The graceful baldacchino over the high altar, which looks out of place in its present surroundings, dates from 1369.

In one of the rebuildings, probably that which was carried out by Pope Clement V, a transverse nave was introduced, imitated no doubt from the one which had been added, long before this, to the Basilica of Saint Paul Outside the Walls. Probably at this time the Archbasilica was enlarged.

more on https://en.wikipedia.org/wiki/Archbasilica_of_St._John_Lateran , https://en.wikipedia.org/wiki/Nave