I was in Paris during my sophomore year of college when I started reading Gibbon's Decline and Fall of the Roman Empire. I made my way through the first volume, immersed in the decadence of the late Roman Empire, an era riveting in its excesses, but at least passingly familiar. Then I entered an entirely new and exotic realm — the eastern Roman Empire, a dominion that survived the fall of Rome in 476 by almost a thousand years.

The Byzantine Empire lasted until 1453, when Constantinople (present-day Istanbul) was conquered by the Ottoman Turks. Through the past two centuries, fabled Byzantium was reduced to little more than a small principality hemmed in by the Ottomans. Eventually, modern Turkey stood in its stead. John Ash is our guide to all these Byzantiums, past and present, in his engaging pursuit of a lost and legendary world.

Ash's epiphany, when it came to Byzantium, was similar, I found, to my own. "It happened," he writes, "on a dull afternoon in school. I had been reading a condensed history of the Roman Empire and was astonished to discover, from a brief final paragraph, that an empire still calling itself Roman had continued to exist until the improbably late date of 1453."

For Ash, a British-born writer now living in New York City, this chance encounter marked the beginning of a deep connection with a remote and beckoning world. "I soon learnt," he writes, "that, in some quarters, this empire had a very bad reputation. It was synonymous with decadence and decline. Even at this early stage it struck me as unlikely that a civilization could have declined continuously for a thousand years. Such longevity surely suggested a people and a culture possessed, at the very least, of great reserves of tenacity and vigor."

Ash is relatively unconcerned with modern Turkey. His passion is the environs of Istanbul, the Anatolian plateau, and the remnants of the Roman Empire of the east. His journey begins in Istanbul, and he quickly makes his way to the Hagia Sophia. One of the oldest, grandest and most intimidating of the world's gloomy cathedrals, it dates from the early sixth century when Emperor Justinian ruled the eastern Mediterranean. "The dimness of Hagia Sophia's interior tends to support the popular idea that the highest aim of Byzantine church builders was an atmosphere of mysterious murk amid which gorgeous images glimmered faintly by candlelight." Still, says Ash, Byzantine builders were actually more interested in a "dazzling illumination," now lost to us under layers of time and dirt.

From Istanbul, we are led to the proximate imperial cities of old — Iznik (Nicaea) and Bursa — and then south, skirting the eastern plateau along the mountains of central Anatolia. Along the way, Ash gradually fleshes out the chronicle of Byzantium. He is particularly interested in the later Byzantine Empire, after the tenth century. The 600 years following the reign of Constantine the Great (who, in the early fourth century, converted to Christianity and so began the widespread Christianization of the West) have received comparatively little attention. The lack of extant monuments from these years is one reason for this elision. Another is Ash's desire to undermine a deeply held prejudice that the later Byzantine Empire, the dominion encountered by the Crusaders, was hopelessly effete and corrupt.

The Comnenid family constitute the heroes of Ash's account. Time and again, he comes back to Alexius I Comnenus (who ruled from 1081 to 1118), his successor John II Comnenus (1118-1143) and Manuel I Comnenus (1143 to 1180). These were the emperors with whom the early European crusaders dealt, and they exemplified a refinement, subtlety, erudition and might at which the crude Europeans could only marvel. Alexius was, in Ash's description, "a master of psychology and political theater." John was "renowned for the justice and benevolence of his rule" and Manuel was "famous for his magnificence and liberality."

The great tragedy of Byzantine history is not, for Ash, the final capture of the city by Mehmed the Conqueror in 1453, but the sack and destruction by the members of the Fourth Crusade led by the Venetians in 1204. The city was burned, the inhabitants killed or exiled, and much of the heritage of the past thousand years was lost. The emperors retreated to exile in Nicaea, and Byzantium never recovered. The Turks and Arabs, in centuries of war with the Byzantines, never acted so brutally.

It's a sobering reminder of how dark Europe's "Dark Ages" were. Though the Crusades may have ushered in the European Renaissance, they did so largely by bringing the truly barbaric Normans, Franks, Britons et cetera into contact with the vastly more advanced civilizations of Byzantium and the Arab sultanates of the Near East. As the Crusaders began their rampages, they fought the Seljuk Turks of Anatolia.

The Seljuks are given respectful treatment by Ash. He visits Konya, their capital and, by the early 1200s, one of the great cities of the Islamic-Mediterranean world and the resting place of the great Sufi mystic Rumi, who attracted both Christian and Muslim followers. Ash lavishes attention on that city's mosques, treasure houses of gleaming tiles and marble. He discovers, too, the Karatay Medrese, a building that once housed a center for Koran studies and today serves as the city's ceramics museum. "The glorious central space," he writes, ". . . is an interior that seems to mirror, with absolute calm, all conceivable intricacies of thought."

From the monumental remnants in Istanbul, to the magical surreal church caves of Cappadocia, Ash traverses the terrain of our forgotten heritage. Ultimately, that is the crucial theme: Byzantium is not foreign.

Upon his return to Istanbul at journey's end, Ash's entry into the dim and beautiful recesses of a church at the city's edge underscores this insight. The church of the Monastery of Saint Savior in Chora (known today as the Kariye Cami), containing extraordinary early 14th-century mosaics and frescoes, stands as a monument to all Ash reveres. "Taken together," he writes, "these [mosaics and frescoes] constitute one of the supreme masterpieces of European art, and deserve to be placed on a level with the nearly contemporary work of Giotto or the greatest achievements of the High Renaissance."

Ash is our armchair guide, and we are transfixed: "Here," he observes, "there is nothing tired or formulaic, nothing remotely decadent or pessimistic. Any unprejudiced observer who steps into the Chora today is likely to be overwhelmed by the brilliance and freshness of the vitrified colors, the shimmering fields of gold, the grace of the figures, the harmony of the compositions and the wealth of picturesque detail. There are peacocks and pheasants, groups of children at play, crags and windswept trees, fantastic architectural backdrops, billowing awnings and nearly cubist views of towns and cities. In John the Baptist Bearing Witness of Christ, a waterfowl grabs at a snake in a pool; in the Annunciation to Saint Anne, a bird flies toward a nest of squawking fledglings high in a tree. There is sorrow in the world of the Chora — mothers lament the deaths of their children, the blind, the crippled and the sick are very much with us--but there is also an intense delight in existence."

In Byzantium, we find a foundation of our culture. Schisms between Eastern and Western Christianity notwithstanding, Byzantine art, architecture, philosophy and theology are woven into the very fabric of Western history. The domes of Italian Renaissance churches come from Byzantine structures; the learning of ancient Greece was transmitted to the West from Byzantium and from the Arab caliphates who in turn inherited them from the emperors and scribes of the eastern Roman Empire; the trappings of autocracy that the crusaders observed at the Byzantine court led eventually to the royal absolutism of the Austrians and the French.

A Byzantine Journey is exotic, but only because so much of our own heritage has been forgotten or distorted. Ash reminds us that ancient Constantinople was not at such a cultural remove from modern Manhattan, with its bustle, complexity, ethnic diversity and crowds everywhere. I loved reading Gibbon. It will always be a great piece of literature. But it is time to lay his mistaken notions of the Byzantine Empire, decadent and dying, to rest.

Zachary Karabell writes from Harvard's Center for International Affairs.