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Exhibit explores Zagreb and its transformations

CAMBRIDGE -- If ever there was a city that never stopped changing, it's Zagreb, now the capital of the Republic of Croatia.

Zagreb's problem is that it's in a location that always seems too near the edge of some bigger place that wants to gobble it up -- Hungary, Turkey, the Soviet Union, Yugoslavia. Such a location makes it a crossroads for trade, but also makes it the prey of its neighbors. Since 1850, Zagreb has undergone 18 major transitions of political leadership, while its population has grown from 20,000 to more than a million.

Zagreb is not a city most of us know, or perhaps have even heard of. But in a fascinating exhibit that's now on the walls of the Harvard Design School, it's worth seeking out. Zagreb proves something we often forget: A city is an organism, as alive as any other ecosystem.

The exhibit is called "Project Zagreb." Curated by Harvard professor Eve Blau , it shows how a little medieval hill town -- the kind of village where you climb outdoor stairs to get to the center -- grew, over the centuries, down a long slope to a river and eventually across it.

The layers of Zagreb's growth, as Blau presents them, are like the rings on a tree. Each one is different. Zagreb has been an example of many styles of architecture and many theories of city planning. In the exhibit, its various eras -- its growth layers -- are embodied in elegant computer-generated models, as well as in photographs, diagrams, and text.

My favorite example of change is the main public space, Ban Jelacic Square . It's named for an equestrian statue of Ban Josip Jelacic , hero of Croatia's struggle for independence from Hungary, which was erected in 1866 . As if in a time-lapse movie, we watch the square change and adapt. In 1930, it sprouts a rooftop electric Bayer aspirin billboard. Then that gets replaced by a huge communist star in 1947, when Croatia was a Soviet satellite.

Then the Soviets dismantle and hide the statue of Jelacic. They replace it with a hideous papier-mache monument to electrification and industrialization. Then in 1956, the Soviet star is replaced by a sign for a pharmaceutical company as Zagreb , now part of Tito's Yugoslavia, allows a first breath of private enterprise to creep back. Finally, in 1991 , the citizens discover the pieces of the statue of Jelacic. They re-erect the equestrian statue. Meanwhile, the pharmaceutical company has been acquired by a U S firm, no doubt a symptom of globalization.

Just as interesting, and just as quick-changing, is a building that looks like a huge carousel for a slide projector. It opens in the 1930s as an art pavilion. During World War II it becomes a mosque, with four new minarets. The mosque lasts only a year. Then the building is the centerpiece of a "Victims of Fascism Square" (goodbye to the minarets), then under the Soviets it is a "Museum of the People's Liberation," then under Tito a "Museum of the People's Republic," then, with independence, a "Museum of the Revolution of the Peoples of Croatia," and today -- once again -- a space for the exhibition of art.

The so-called Zagreb Fair is the subject of another chunk of the exhibit. A local tradition, during the Cold War it became one of the places where goods from the United States and the USSR -- everything from fashionable clothing to industrial products -- were displayed side by side. It was a sort of World's Fair, and its national pavilions, too, are an index of changing fads in architecture.

I'm not going to pretend that "Project Zagreb" is easy going. It's dense and, at times, it indulges in some heavily academic language. It's demanding but it's an exhibit that will repay any amount of attention you want to give it. There's always something more to be discovered here.

Eve Blau, the curator, is the author of a classic book "The Architecture of Red Vienna," among others. "Project Zagreb," too, will become a book. Blau has one ambition for both the show and the book. She wants them to demonstrate that we should think of a city -- and we should plan for it -- as an ever-changing process over time. A city is always in transition. It should never be thought of as a static formal object.

Robert Campbell is the Globe's architecture critic. He can be reached at