It was a picturesque sight loved by romantic 19th-century European travelers to the Holy Land -- a rugged medieval castle on the Mediterranean coast, its walls in ruins, waves breaking on a strange jetty of marble shafts laid horizontally and extending into the sea.

Late 20th-century archeologists have made systematic sense of the scene and its haunting juxtaposition, with results that are every bit as romantic and more vivid, more compelling. Most likely, they say, the crusaders who built the fortress in the 12th century also fashioned the crude jetty out of columns ransacked from an ancient classical city there.

"King Herod's Dream: Caesarea on the Sea," an exhibition at the Smithsonian's National Museum of Natural History, offers a portrait of that forgotten city across more than 15 centuries, an engrossing tale of climactic, frequently bloody, changes in religion, philosophy, politics and city life, and of notable consistencies, too.

The efficaciousness of contemporary archeology is a subtheme of the show, which is not, by and large, one of stupendously beautiful ancient objects (though there are some). Since 1971 more than 3,500 volunteers have joined two international teams at the site in Israel -- the Joint Expedition to Caesarea Maritima (JECM) and the Caesarea Ancient Harbour Excavation Project (CAHEP) -- to patiently uncover, date, catalogue and interpret buildings, walls, streets, artworks, engineering wonders and thousands of homely artifacts bespeaking the once vibrant life of the place.

Caesarea Maritima, like most cities, was a palimpsest of cultures, but it had an extraordinary beginning under King Herod, who ordered it built in the 1st century B.C. and who supervised its construction during a mere dozen years. It also had a definitive end in 1291, when the Christian citadel was dismantled at the command of a Mamluk sultan who wanted no more trouble from the west.

Before it was Caesarea, however, it was Strato's Tower, a Greek trading settlement on the busy shipping route between Egypt and Phoenicia. Archeologists of the two teams, interpreting evidence differently, disagree about the size and importance of this settlement, but they come together in awe of Herod's feat.

Yes, this is the Herod of the slaughter of the innocents recorded in the New Testament, an unsavory, paranoiac despot to be sure. "I would rather be Herod's pig than his son" is the justifiable witticism attributed to Caesar Augustus, the king's indispensable Roman patron after whom, predictably, Herod's city was named.

But he also was a skillful general and a canny, intelligent politician who used his power and vast wealth to build for his own greater glory and that of Rome, and in order to solidify his sometimes shaky rule. He reconstructed the Second Temple in Jerusalem, for instance, in an attempt to appease the resentful Jews in his domain, and he built numerous amazing fortress-palaces throughout Judea.

Caesarea, though, was his ultimate architectural triumph -- a planned port city completely built and populated at his behest. The artificial harbor, named Sebastos (Greek for Augustus), was an amazement to his contemporaries. An ancient text describes how, "by lavish expenditure, the king conquered nature herself, constructing a harbor larger than the Piraeus {the harbors of Athens}, and providing deep anchorages in its innermost recesses."

For centuries this account was thought to be outlandish hyperbole but, thanks to the CAHEP team's underwater excavations, it turns out to have been true. Using Roman cement and hordes of slave laborers and professional divers, Herod's engineers (also probably Roman) sank great foundations, piers and slabs and built upon them an elaborate system of breakwaters, wharves, towers, vaulted warehouses and free-standing columns supporting symbolic statues.

The walled city itself was no less resplendent. Organized on a regular grid of streets, according to then-prevalent Greek models, it contained all the accouterments of Roman power and civilization -- a theater, a marketplace, prominent public buildings, baths, palaces, aqueducts, sewers, paved streets lined with colonnades. That so magnificent, but thoroughly artificial, a creation thrived through six centuries is due partly to its location on sea lanes in the productive Plain of Sharon, but more importantly to imperial support, first from the Romans and then, after Emperor Constantine's conversion in 313, from the Christian Byzantine empire.

Like that of many port cities, Caesarea's population comprised a rich stew of ethnic, religious and economic groups. Through much of its heyday these lived side by side in relative, if enforced, harmony -- one of the great pleasures in this exhibition is to imagine the vitality of the great city under the Mediterranean sun. But intolerance was common enough -- Christians died in its amphitheater, Samaritans were executed by the Byzantines, and the size of the city's population in the century after Herod has been estimated by the grisly slaughter of 20,000 Jews by Roman legions in A.D. 66. In later years, of course, Christians and Moslems killed each other with great zeal.

Caesarea's most significant metamorphosis occurred in the 7th century with its conquest by the Moslems, who allowed the port to disintegrate. Over the years the city's new inhabitants transformed the Greco-Roman pattern of streets and monumental avenues into the appealing but decidedly different texture of the medieval Islamic suq -- a city filled with bustling, tightly packed markets and houses. A 10th-century traveler wrote that "on the coast of the Greek sea there is no city more beautiful, nor any more filled with good things."

Under the crusaders, though, Caesarea became less a city than a military outpost of feudal, European civilization, its days, in retrospect, clearly numbered. Through all of its transformations, however, certain constants remained. Herod's city was dominated by a great pagan temple, built upon a platform with a dramatic command of the harbor. In succeeding eras the same platform was used for a Byzantine church, a splendid mosque and a cathedral.

The exhibition, be forewarned, is rather too compactly installed. Maneuvering its alleyways will be difficult on weekends. But all involved deserve credit, especially the archeological teams and the Center for Archeology at the University of Maryland, which took the lead in organizing the show and an excellent book. Circulated by the Smithsonian Institution Traveling Exhibition Service, the show departs Washington on June 19.