THE DECLINE AND FALL of the Roman Empire has been the subject of much good literature, from Gibbon's six-volume history to Auden's tiny brilliant poem, "The Fall of Rome." It's not hard to see why. That long decline and eventual crash is not only one of the supreme dramas of history, it is one charged with special significance for the 20th century. It is in the West, anyway. We wonder uneasily if our own decline began in 1914 (if we are Europeans) or somewhere around 1965, if we are Americans. We may even look around for a Caesar or a president who will assure us it hasn't.

Bryher's novel Roman Wall is thus part of a long tradition. But it differs from most other books about the dying empire in one very striking way. Most of them give some kind of overview. They tell the story from the center, as the CIA would have known it, if the Romans had had a CIA. Consuls and tribunes and vast armies play their splendid parts.

Rome is barely mentioned in Roman Wall. The great city in this book is a place called Aventicum, which most of us have never heard of. (It survives to this day as a Swiss village of a couple of thousand called Avenches, which most of us have never heard of, either.)

It is historic fact that in 265 A.D. the German tribes-- the Alemanni--crossed the Rhine and captured the Helvetian frontier city of Augusta Raurica. Half a legion perished in one day. Pausing only to loot and burn, the tribes moved on to besiege Aventicum, the provincial capital. That also rapidly fell. Its garrison died in the burning temple. The city was never rebuilt, though the mere ruins remained enough to impress Byron 15 centuries later.

Around this minor incident in a minor Roman province (Gibbon never mentions it), Bryher has written a wonderful novel. Reading it, it is hard to remember that she is actually a woman still living, and not an eyewitness to those far-off events.

The story has two sets of main characters. One set consists of a Roman centurion named Valerius, his sister Julia, and their small household (two free-born Helvetians and several slaves). Valerius has not had a successful career. As a very young officer, 14 years earlier, he had an affair with the wife of his commanding officer, and got caught. Banished to the frontier, he now commands an outpost of only 15 men. He and Julia couldn't even afford the modest villa they live in if it weren't for the fact that real estate prices are steadily dropping in that part of Switzerland as the threat of invasion increases.

The other set consists of the governor of the province, his wife-in-all-but-name, and several members of his far larger household. And linking the two sets, a traveling Greek merchant named Demetrius. Demetrius spends his winters in Verona, but each spring he sets out with a train of mules and slaves to sell amber, furs, perfume, and other luxuries in the provinces. He is a good man: honest, very intelligent, by the standards of the third century kind to his slaves. He also, you gradually learn, gets his best furs from the Alemanni, paying for them with Roman weapons. That practice is totally forbidden, and quite widespread.

Since the entire story takes place in the year that Aventicum fell, it is obviously an exciting one. It would make a good movie--I can think of 20 gloriously filmable scenes without even trying--and perhaps someday a smart producer will take notice.

But plot is the least of the book's strengths. More impressive is the extraordinary texture of reality. There is not one character whose actions and thought processes do not seem to me at once alien (these people lived 1,700 years ago) and true. Take Valerius the centurion and his sister Julia. They are conservatives, imbued with the old-Roman ideas of dignity and propriety. Valerius is prepared to die uselessly at his outpost unless he gets orders to evacuate, even though he knows the governor is a heavy drinker and very apt to forget details in a crisis. Julia, a beautiful woman around 30, will not marry the high-ranking officer who seeks her, because she feels tainted by her brother's disgrace. But at the same time Valerius is a comfort-loving man who must have his nap after lunch and his bought Swiss girls, and also a true mystic whose long-ago affair was as much religious experience as it was sexual consummation. Julia is neurotic--an anal-retentive in Freud's terms-- scrubbing the mosaic floor in the kitchen of their villa until she actually dislodges tiles; she is also a woman of deep power, drawn to the austere rites of Apollo.

Or take Felix, the one Christian in the book. Felix is the overseer for Demetrius' trade caravan, a slave for most of his life who has recently managed to buy his freedom. (Demetrius had to pay the government a tax on the purchase price.) Felix is a convert, a true believer. But he is also a very bad Christian by modern standards, because he is so full of hatred. He is not about to forgive anybody, especially not Romans. When he goes to confession in Verona in the winter, he has to--and does--lie to the priest about his feelings, in order to get absolution. These are complex characters.

More impressive still are the ways in which Bryher shows the powerful but decaying society in which these characters move. Shows, not discusses. She never either preaches or teaches. Rather, the decadence emerges in glimpses such as Demetrius meeting Alemanni chiefs in secret, or a well-bred youth being ostentatiously bored at the gladiatorial games in Aventicum ("two Thracians slashing at each other")--he is too civilized to take the empire seriously.

And most impressive of all, Bryher manages to convey a subtle sense of why the empire is beginning to decay. She does not, like Gibbon, pin it on Christianity; what she suggests is that the sheer size of the empire is mainly responsible. The virtues that made early Rome great are by no means lost; they are just increasingly out of place in vast anonymous world-power Rome, even as the old virtues of participatory democracy seem to be increasingly an impediment in vast anonymous world-power America. (This is my comparison, not hers. The characters in Roman Wall sometimes look back from 265 A.D.; they never cheat and look ahead.)

Bryher is really an 87-year-old Englishwoman named Winifred Ellerman. She took her pen name when she first began to write, in part because her father, Sir John Ellerman, Baronet, was a power in English publishing, and she scorned to trade on that fact. She has been poet and avant-garde film critic as well as novelist. But her special genius is for portraying ancient societies as they died, or perhaps were painfully reborn: Gate to the Sea is a fine example, an almost magical evocation of the conquered Greek city of Poseidonia around 300 B.C. But Roman Wall is her masterpiece.