Updated: Feb 28, 2021
Here, I continue to point out some features of church architecture, generally before 1100. Please see Part 1 post for more on this topic.
San Lorenzo Fuore Le Mura
San Lorenzo is a special case in terms of its architecture, because it is a hybridization of two very early churches which were side-by-side in the 6th century. I include it here for two reasons: it's an example of how, as churches were renovated and expanded later in the Middle Ages, the older section would be preserved in part; it retains its marble bishop's throne behind the altar. In San Lorenzo, as with San Costanza, Sant'Agnese, and Santo Stefano, the presence of the saint's tomb is the focal point of the worship. Various scholars point to the origins of Christian worship in catacombs, over the tombs of saints and fellow Christians, as a beginning of this tradition.
By the 4th century, when public churches become part of state religion, altars were customarily placed over the tombs of martyrs, and moving relics to appropriate sites, or, raising churches on sites of martyrdom became standard practice. In the case of San Lorenzo here, you see a response to the great resurgence of pilgrimage and church power in the 12th/13th centuries. The crypt, where the martyr was originally worshipped, becomes a lower floor, and a more grand church is literally raised up. The 6th century floor is raised in San Lorenzo, becoming the sanctuary. In the foreground you see the fashionable Cosmatesque large-pieced mosaic floor, a series of carpets in stone. The tomb of the saint is no longer the altar, but is under the altar. Other churches which still have this element include the duomo in Modena and Canterbury Cathedral.
The upshot of this is that the churches themselves are identified by their name-saints, as sites where that saint (through the spiritual resonance of their relics) can be prayed to for intercession. Performing mass over a saint's tomb reaffirms the belief in resurrection. The architectural decision at San Lorenzo to keep access to the crypt visible from the new nave, and to raise the early floor to be the holy sanctuary sets a stage for the ritual theater of the liturgy.
Please note the marble benches with colored marble and gold inlays in the background of the photo above, in particular the raised marble throne, the cathedra. This is where the bishop would have sat at particular points in a ritual or meeting with other officials in the sanctuary. I have a better image of one in San Vitale in Ravenna.
San Vitale Cathedra
There are other famous cathedra thrones of this type, perhaps most notable is the Throne of Bari.
But, back to San Lorenzo.
There are two side benches of marble, flanking the bishop's seat. Much less fancy, but probably would have held cushions. The 13th-century lion is taken from its orginal place in the portico entrance.
Speaking of the portico, the 13th century facade has the remnants of beautiful frescoes, my favorite of which is this enthroned Mary, with her enigmatic gaze. I am stunned that more isn't done to protect the piece in the open air. She's crumbling away before your eyes.
The 9th century church, Santa Prassede has an original program of mosaics which contain several standard images from Byzantine iconography, many of which are used less often in the later Middle Ages. You see here two arches. the one in the foreground is a triumphal arch (sometimes called a "victory arch"), which is an interesting adaptation of classical Roman architecture to Christian purposes. Where the Roman triumphal arch was built outside, to commemorate a great leader, especially if he won a particular military campaign, the Christian triumphal arch is an interior feature. Roman polytheism built temples with small interiors, but large and ostentatious exteriors--worship and rituals were mostly performed outside the building. Christianity inverted the practice (perhaps rooted in their origins of having to worship in private homes and underground in catacombs), with the exteriors of churches being rather plain, but the ostentation and the rituals taking place indoors. Just so, the interior victory arch celebrates the victory of the Church Triumphant (Christ and the Saved in heaven), and sets the building itself as a ritual space for the Church Militant (those who battle sin and evil on earth--the clergy and the congregants).
So, this first arch shows celestial Jerusalem, (presented as a bejeweled walled city), with the apostles (front row, in white robes, carrying their crowns), flanking John the Baptist (in a "purple" robes), Mary, and on the other side of Jesus, Santa Prassede. Jesus is on a raised platform, as befits a king, and has angels attending him. (I'm not sure about the two white-robed figures standing above and behind the apostles.) On the far sides, you see angels guarding the gates of the city. The underside of the arch has at its apex the name of the (then-living) Pope Paschal I), who commissioned the church.
The second arch is an apsidal arch (it frames the apse, which is the convex half-dome which is behind the sanctuary and altar, and which usually has an imposing religious scene). In it, you see Christ as the sacrificial lamb, enthroned, per the Apocalypse, with a scroll bearing the seven seals on the footrest of his throne. On either side of him, the seven candles, representing the seven churches of Asia mentioned in the Apocalypse, are depicted.
Santa Prassede Halos
When you view the apse itself, you see Christ in the center, with the River Jordan behind him, and separating him from the saints. Peter and Paul are closest to him, and they each are presenting a newly-crowned saint: Santa Prassede, and her sister, Santa Pudenzia (who has her own church nearby). What's really remarkable in this mosaic is the figure on the far left: Pope Paschal 1. He is shown presenting the church of Santa Prassede itself to Jesus and the saints. This is significant because he has a rare square halo; he has it (a halo type reserved for a living saint--and, remember, most saints are so because they are martyred; that is, dead) because he was canonized while alive. Above him on the palm tree is a phoenix (with its own very striking halo), symbolizing eternal life. Paschal was apparently pretty convinced that this church was his ticket to heaven.
The Tetramorph in Mosaic Form
Mark and Matthew
Santa Prassede's arch has a gorgeous array of the tetramorph--four creatures which are typically described as representing the four synoptic gospels (and, indeed, each is carrying his book), but they are also a disambiguation of an older apotropaic creature from the East, the Lamassu. The Lamassu was a protecting creature which protected in dominion over all (The heavens--the eagle; the domesticated realm--the bull; the wild realm--the lion; the human realm--the man. Not all lamassu have lion parts, so the bull in those represent all animals.). The gospels and their evangelists as the tetramorph affirm God's total rule. In the image above, you see Mark (the lion) and Matthew (the man/angel). I love going through medieval art to see the various weird depictions of lions by artists who'd never seen one in reality. Below Mark and Matthew you see saved martyrs with the crowns of immortality they receive in heaven (per Revelations). It is a funerary crown (used in burial practices for pre-Christian royalty), reinvented/transformed into a crown to signify having won eternal life. It's useful to note here that these martyrs all have the same facial features--their individuality is not important in this period and type of art--what they share in common is what's important. They are as similar as a representation of soldiers would be.
Below, you see the other two figures of the tetramorph--John, the eagle, and Luke, the ox.
Luke and John
I will include here a video which will give you a fuller view: