
Via that wonderful Roman site, Eternally Cool.
Exploring Zenobia's World. The Incredible Rise and Fall of the City of Palmyra
on the road to Roman territory, Zenobia, wife of Odenathus, who was ruler of the Saracens in that district, once founded a small city in earlier times and gave her name to it; for the name she gave it was Zenobia, as was fitting. (Buildings 2.8)

By the side of Zenobia, Procopius continues, flows the Euphrates River, passing to the east of it and coming very close to the circuit-wall on that side; but since high mountains rise beside the river at this point, the stream cannot spread out at all, but by reason of the proximity of these mountains and because it is constrained by its banks, which are hard, it would gather its stream into an extraordinarily narrow space

Al-Zabba [Zenobia] built herself a fortress on the western bank of the Euphrates. She used to spend the winter with her sister, and the spring at Batn al-Najjar from where she would go to Palmyra.The connection with Palmyra makes it clear that Tabari's Al-Zabba is the same as our Queen Zenobia. Later, Tabari says that she also built a castle beyond the Euphrates with a tunnel under the river to link the two banks.
The long period of time that had elapsed since [Zenobia's foundation of the city] had reduced its circuit-wall to a ruin, since the Romans were quite unwilling to take care of it, and thus it had come to be altogether destitute of inhabitants. So it was possible for the Persians freely, whenever they wished, to get into the middle of Roman territory before the Romans had word of the hostile inroad.
The great period of Zenobia's expansion came under the Emperor Justinian (527 - 565) in a renewed effort to fortify the Euphrates against the continuing Persian threat. Justinian provided it with massive defences around 545 CE -- and this is basically what we see today.He also found that portion of the city's circuit-wall which faces the north dangerously weakened by the passage of time; so he first took it down, along with the outworks, clear to the ground, and then rebuilt it.At the same time, the emperor gave the city of Zenobia all the trappings of a Byzantine-Roman town: there are still traces of a north-south street (the cardo maximus), several baths (complete with frigidarium, tepidarium, and caldarium), two Christian basilicas, a gymnasium, and a forum.
[The] Emperor Justinian rebuilt Zenobia completely and he filled it quite full of inhabitants, and he stationed there a commander of select troops and a thoroughly adequate garrison, and made it a bulwark of the Roman Empire and a frontier barrier against the Persians; indeed he did not simply restore its previous form, but he actually made it very much stronger than it was before.
The Praetorium (left), about halfway up the hill, has vaulted rooms surviving even today up to three storeys high, and a very large hall. It is built of blocks of local white crystalline gypsum. I don't know if anyone has estimated the number of troops that could be quartered there, but it was undoubtedly a serious commitment, as Procopius said.
Footnotes and Illustrations credits:
An expert in ancient timekeeping thinks we can tell the time from this sunbeam entering the Pantheon in Rome. The building is, he says, a colossal sundial.The gods confound the man who first found outLike those unutterably smaller dials, the Pantheon's dome forms a perfect hemisphere inside. Hannah reckons that this is no coincidence but, he says, "a deliberate design feature."
How to distinguish hours! Confound him, too,
Who in this place set up a sun-dial,
To cut and hack my days so wretchedly
Into small portions.
You enter through gigantic bronze doors – the originals. There were once veneers of precious marbles within, pure gold tiles on the roof, and the bronze doors, weighing twenty tons each, were themselves once covered with plates of beaten gold. Inside the temple is one immense circular room. The interior is a cylinder above which rises the hemispherical dome, constructed of stepped rings of solid concrete with less and less density as lighter aggregate (pumice) is used as it rises. The only natural light enters through the oculus at the centre of the dome and through the bronze doors to the portico.At equinox, the sun is on the celestial equator -- where Earth's equator would lie if projected into space -- which was seen as the most stable part of the sky, a perfect eternal home for the gods.Also [Hadrian] completed the building called the Pantheon. It has this name, perhaps because it received among the images which decorated it the statues of many gods, including Mars and Venus; but my own opinion of the name is that, because of its vaulted roof, it resembles the heavens.(53.27)
The interior space was designed around a perfect sphere 43.3m (142 feet), in diameter, the largest dome ever constructed in masonry. Its mathematical perfection is underscored by its symmetry: the width and the height of the dome are equal. A whole sphere can be inscribed in the interior volume, with the diameter at the floor of the cylinder of 43.3 m (143 feet) equalling the interior height."this open and secret temple, conceived as a sundial. The hours were to circle the center of its carefully polished pavement where the disk of the day was supposed to rest like a golden buckler; there the rain would make a limpid pool from which prayer could transpire like smoke toward the void where we place the gods.”All other Roman temples before and since cope perfectly well without the oculus solution. Even Hadrian himself designed the enormous temple to Venus and Rome without such a device. Hannah thinks that by marking the equinoxes, the Pantheon was designed to elevate emperors who worshipped there into the realm of the gods.
A grille above the door allows a sliver of light through to the front courtyard -- the only moment in the year that it sees sunlight if its main doors are closed. So, it seems certain that the building served some sort of important function on the equinoxes. Extinguish the torches ... and it's easy to imagine that, twice a year, the emperor would get to stand in that ray of light.
Illustrations
Top: From Great Buildings.com
Upper left: From The Monolithic Dome
Middle left: From Roman Pantheon
Below: No place like dome: the Emperor Hadrian's connection with the Pantheon at TimesOnLine