Since it was originally built, only two things have happened to the structure that are visible today. The first happened abruptly and very soon after the arch went up: Septimius died of natural causes while on a military campaign at York, England, in February of 211 AD. In December of the same year, Carcalla's second attempt to have Geta murdered succeeded. It was a real family scene featuring Geta pleading for mercy in the arms of his distraught mother, Julia Domna, as Caracalla and his henchmen cut him to ribbons. According to ancient historians, as many as 20 thousand of Geta's supporters died in the two-week massacre that followed. (Julia Domna, survived and, although she was kept on a short leash, she became one of Caracalla's advisors.) At any rate, after the mess was cleaned up, the inscriptions on the arch had to be amended to remove all mention of Geta. With a good pair of binoculars you can still see where the fastenings for the long-gone inlaid bronze letters were changed in the fourth line on each side.
The second change to the arch was gradual: the bright colors, which made all the fine decorations visible to viewers on the ground, faded or flaked away. We know that ancient Rome was not the white marble city that you might expect from seeing the ruins or the Hollywood reconstructions. The arch of Septimus Severus, like the Forum monuments that surrounded it, would have been dazzling with primary colors and with gold, silver, and bronze inlays and overlays. The 1999 cleaning of the monument, while necessary to prevent additional damage from pollution and mildew, reduced further the ground visibility of some architectural and sculptural elements: the soot and mildew had to some extent highlighted the details, and cleaning reduced everything to the cream white of the Pentelic marble of which the arch is constructed.
The ground level around the arch has fluctuated a great deal in the almost 1800 years since the arch went up, but now it is almost exactly the same as it was when the arch was built. Originally, there was a stairway leading up to the base of the arch -- the road through the arch wasn't built until Medieval times.
The arch is 23 meters high, 25 wide and 11.85 deep. The central archway is 12 meters high and 7 wide, and the side archways 7.80 meters high and 3 wide. Between the central and side arches are vaulted passages with coffered ceilings. On each face of the arch are four free-standing fluted columns 8.78 meters high and 0.90 meter in diameter (at the base) standing on projecting pedestals. Behind them are corresponding pilasters. An entablature surrounds the arch, and above it is the lofty "attic" story, 5.60 meters in height, within which are four chambers.
Over the side arches are narrow bands of reliefs representing generic triumphs of Rome over conquered peoples. Above them are four larger reliefs which show the campaigns of Severus in the East, and these may well have been copied from painted panels displayed before the arch was constructed. In the spandrels of the central arch are winged Victories, and in those of the side arches there are river gods. On the keystones of the central arch are reliefs of Mars Victor, and on the pedestals of the columns, Roman soldiers driving captives before them. Coins of Severus and Caracalla show that there was a six- or eight-horse chariot, in which stood Severus and Victory, escorted by Geta and Caracalla, on the top of the arch. Coins also show four additional equestrian figures at the corners. None of these statues have ever been found.
So why did this arch stand when all the others fell? Most sources agree that it was because this particular arch had an early owner: it was built into the fabric of an old church and fortified cloisters dedicated to Ss. Sergio e Bacco. The Church and cloisters were removed to another location before the Renaissance but the church retained ownership of the arch. And then the Arch became a tourist attraction in its own right, and so its marble was not mined to build Renaissance churches and Palazzi. Until the 19th century, the bottom half of the arch was buried in accumulated soil and rubble.
with old prints and photos at http://www.siba.fi/~kkoskim/rooma/pages/FSEPTIMI.HTM and
Septimius Severus bio info:
(from Historia Augusta -- 4th Century AD)
Julia Domna bio info:
Caracalla kills Geta: http://www.uky.edu/ArtsSciences/Classics/wlgr/wlgr-publiclife180.html
P.S. 2: Pentelic marble is quarried from the sides of Mt. Pendelikón north of Athens.
P.S. 3: Haven't you ever wondered where the English word "attic" came from? An "attic" story was an additional monumental level, often with short facing columns or pilasters and interior rooms, built above or as part of the entablature that surmounted and joined columns or arches. On triumphal arches, the "Attic" provided highly visible faces for inscriptions. The architectural style originated in Athens (Attica) in Greece.