
Your not much of a history buff, I take it.
The Roman Missal was revised on a number of occasions after 1570. Pope Clement VIII made a general revision, as did Pope Urban VIII and other popes.
Almost all Latin-rite altars of recent centuries were accordingly built against a wall or backed by a reredos, with a tabernacle placed on the altar or inserted into the reredos.
There were exceptions to this orientation, however. The Tridentine Missal itself speaks of celebrating “versus populum” (facing the people), and gives corresponding instructions for the priest when performing actions that in the other orientation involved turning for him to face the people (Ritus servandus in celebratione Missae, V, 3).
In several ancient churches in Rome, it was physically impossible for the priest to celebrate Mass facing away from the people, because of the presence, immediately in front of the altar, of the “confession” (Latin: Confessio), an area sunk below floor level to enable people to come close to the tomb of the saint buried beneath the altar.
The best-known such “confession” is that in St Peter’s Basilica, but several other churches in Rome have the same architectural feature, as for instance the Church of the Four Crowned Saints in Via dei Santi Quattro, which has the apse to the east. It is said that the reason the Pope traditionally faced the people when celebrating Mass in St Peter’s was because, due to the difficult terrain, the basilica was built with its apse to the west. “For whatever reason it was done, one can also see this arrangement (whereby the priest faced the people) in a whole series of church buildings within Saint Peter’s direct sphere of influence.”
The 2002-2006 excavations at the Basilica of Saint Paul Outside the Walls have shown that the original basilica there, built, like the Basilica of Saint Peter, at the time of the Emperor Constantine I, also had its apse to the west, although there were no constraints of terrain to impose this orientation.
The same orientation is seen in more of the oldest churches in Rome, confirming the statement that, before the sixth century, churches were usually constructed with the entrance to the east:
"When Christians in fourth-century Rome could first freely begin to build churches, they customarily located the sanctuary towards the** west end** of the building in imitation of the sanctuary of the Jerusalem Temple.
Although in the days of the Jerusalem Temple the high priest indeed faced east when sacrificing on Yom Kippur, the sanctuary within which he stood was located at the west end of the Temple. The Christian replication of the layout and the orientation of the Jerusalem Temple helped to dramatize the eschatological meaning attached to the sacrificial death of Jesus the High Priest in the Epistle to the Hebrews."
The people originally stood in the side aisles and for the first portion of the service faced the altar but at the approach of the consecration they all turned to face east towards the open church doors, the same direction the priest faced throughout the Eucharistic liturgy.