The First Man in Rome
The Grass Crown
Fortune's Favourites
Caesar's Women
Caesar
The October Horse
Colleen McCullough originally set out to document the fall of the Roman Republic; six massive books and several years later, she has accomplished that and more.
The series starts about two generations before Julius Caesar, and documents the sweep of events that took place in the lives of Rome's movers and shakers, up until the end of the Republic. It begins by focussing on Gauis Marius, a country squire with great military talent but no ancestors (vitally important in Rome's snobbish Senate), and Lucius Cornelius Sulla, a man with a wealth of the best patrician ancestors but who lives in utter penury, depending on his stepmother and mistress for his monetary needs, a fact which galls him deeply. Both men marry daughters of Gaius Julius Caesar (grandfather to the famous Caesar), and thereby assure their acceptance into the society of Rome's upper strata.
While the focus stays on Gauis Marius and Lucius Cornelius Sulla, and their ultimate effect on Rome during their rise and fall, the books include a wealth of information about the customs and rituals of Roma herself, and broadens to encompass other countries in Asia Minor and Gaul, and the characters that intrigue for power. Rome is threatened by hordes of Germans; Rome is threatened by pirates in Mare Nostrum ("Our Sea"); Rome is ultimately most threatened by her staid Senators who cannot admit that they are hopeless generals, constantly losing battles through sheer arrogance, and unable to care more for Rome's citizens than for their own clout and ambition. This leads to the rise of a new kind of senator-general: the charismatic man who is more than willing to overturn Rome's timeworn traditions for his own benefit and thus throw her into economic and military turmoil.
Another force that overturns the age-old oligarchic rule of the Senate is a new breed of tribunes of the Plebs, ten men elected each year having the power to enact laws (plebiscites), and holding another incredible power - the veto. These tribunes of the Plebs, traditional enemies of the patrician class, start to fill the power vaccuum in the Forum by seeking to win over the people with demagoguery, "Panem et Circenses" (bread and circuses) in order to rule Rome from the bottom up.
As one generation passes away, a new one grows to fill its shoes: Aurelia, wife of Gauis Caesar and mother of the Gaius Caesar; Marcus Livius Drusus, perhaps the greatest of Rome's Plebeian Tribunes, and his sister, Livia Drusa; Quintus Servilius Caepio, one of the richest men in Rome, and Drusus' brother-in-law; and the various lesser characters who go to weave a tapestry brilliant with colour and life. These all serve as background to the major players in the last books: Marcus Porcius Cato, Caesar's lifelong enemy, the son of Livia Drusa; Servilia, Cato's half-sister, Caesar's long-time mistress and mother of the famous Marcus Junius Brutus; Marcus Antonius, Caesar's cousin and rapacious, brawling despoiler of Rome; and, of course, the great Gaius Julius Caesar himself.
Ultimately, after the first three books, the focus of the narrative shifts from a huge agglomeration of characters and countries towards the life and deeds of one man, Gauis Julius Caesar. A child in The Grass Crown, and a young man in Fortune's Favourites, he grows to maturity in a Rome full of political and military turmoil, fought over by her generals and wracked with colossal changes. We see Caesar's rise in Caesar's Women, his brilliant campaigns in Gaul in Caesar, and his election as perpetual Dictator of Rome after the great battles of Pharsalus, Thapsus and Munda. The October Horse then deals with Caesar's relations with Cleopatra, his murder, and the rise of his nephew and adopted son Octavianus, later to become Caesar Augustus, Rome's first Emperor in all but name.
Throughout the series, the author documents the life-styles of Romans and Greeks and Gauls, giving a feel for the period and a sense of how ancient minds worked and ancient societies interacted. She does an amazing job. One is hurled headlong into a world where human life is cheap: wives can be executed by their husbands with no legal recourse, and children by their parents; slavery abounds. One witnesses horrifying genocides, wars and murders, and comes to the realisation that human nature has changed not at all. It is possible to watch, step by step, the fall of the Roman Republic and the rise of the Empire, trying to understand why it was inevitable, and how the sweep of political forces brought Rome ultimately to the time of Augustus. Each book is well worth reading on its own, but they work best in sequence, and I cannot recommend them more highly.