In some ways there is little of surprise here; it is a classic Roman basilica as one would expect. However it is in some of the details shown in these that lay some of the particular interest.
Of course, even these details, which are architecturally focused, still only give a vague insight into what it must have been like to enter the splendour of the Constantinian basilica when one considers some of the ornaments found within which are detailed in the Liber Pontificalis.
This includes ornaments such as:"...four brass candlesticks, 10 feet in height, overlaid with silver, with figures in silver of the acts of the apostles, weighing each 300 lbs. ... a golden crown before the body, that is a chandelier, with 50 dolphins, which weighs 35 lbs.; 32 silver lamps in the body of the basilica, with dolphins, weighing each 10 lbs.; for the right of the basilica 30 silver lamps, weighing each 8 lbs. ; the altar itself of silver overlaid with gold..."
For these we will have to use our imaginations and in doing so we should not only imagine the beautiful and precious metalwork described, but also the living, warm flickering glow of the lamps which were so eloquently described by the fourth century Christian poet, Aurelius Prudentius Clemens, in his poem, Hymnus ad Incensum Lucernae, where he speaks of the atmosphere of these paleo-Christian churches:
Ceilings fretted with gold, gleam with brilliant light
Shed from pendulous lamps swaying on supple chains;
The flame fed by the oil languidly swims about,
Casting flickering rays through the translucent glass.
With all that in mind, here are some of the digital recreations I recently came across. The first shows the interior of the basilica taken from the nave, and here I would also cross reference you to the first image shown at the beginning of the post. Here we see the classical basilica form, lined by various coloured marble columns. Between these columns one can make out some of the aforementioned lamps.
A closer view of the nave and altar with the confessio beneath:This next illustration shows some of the marble detailing that may have been found on some of the outermost, interior walls.
For those for whom this is a new subject, the following shows a reconstruction of the original exterior of the original Constantinan basilica of St. Peter's in Rome.