Scholars of ancient history and other archaeologists were either unfamiliar with the Gabii findings or cautious in their comments. Richard J. A. Talbert, a historian at the University of North Carolina at Chapel Hill and scholar of ancient geography in the Mediterranean world, visited the Gabii excavations last year. Dr. Talbert noted that in later Roman tradition, Gabii was seen as “a source of ideas and culture.” But he added, “We really don’t have enough evidence yet to say how influential Gabii was on Rome in these early periods.”

Christopher Ratté, the director of the Kelsey Museum of Archaeology at Michigan, said the excavation was part of a concentrated examination of the social environment in central Italy before the rise of Rome as a world power. “It has been quite a surprise,” he said, “to find that it was still possible to break new ground like this in a region that has been so well researched.”

The Gabii Project, supported in part by the National Endowment for the Humanities and in collaboration with the Italian Archaeological Service, had explored and mapped the more than 170 acres of the ancient city, which was built on the slopes of an extinct volcano where the crater had become a lake. Then the archaeologists encountered the elaborate building complex. Dr. Terrenato said he was immediately struck by the size of the stone blocks in the retaining wall on the slope inside the complex. Each one weighed thousands of pounds.

“This was like Lego construction,” he said. “They were stacked one on top of each other without any glue binding them together. This is the only technique they had access to, and it must have been the desire for this kind of grand construction that drove them to the invention of mortar about 125 years later.”

The researchers also admired some of the architectural details: rows of stone pillars, courtyards and terraces covered in mosaic tiles in geometric patterns, a 21-step staircase cut into bedrock. They said this showed that the people were beginning to experiment with modifying their natural environments — cutting back the natural slope and creating a retaining wall.

“All this only whetted our appetites,” Dr. Terrenato said as he looked ahead to next summer’s continued excavations and a hoped-for extension of the project beyond 2014.