Here’s what you’ll learn when you read this story:

• Researchers tracked Celtic silver coins to a mountaintop site in Croatia.
• After years of excavation, archaeologists from the University of Zagreb located a 3,200-year-old fortress with stone defenses.
• The northeastern Croatian site is a “jackpot” of a discovery, according to the experts.

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When a hoard of looted Celtic silver coins popped up on the black market thanks to some metal detectorists, researchers at the University of Zagreb took notice. The team tracked the site of the coin’s discovery to the top of Mount Papuk, roughly 2,000 feet above sea level, and spent two years in search of a reason for why that spot was chosen to stash a hoard of coins. What they found was an astonishing 3,200-year-old stone fortress.

“At this place, we did not expect anything like this,” Hrvoje Potrebica, a professor of prehistoric archaeology at the University of Zagreb, said in a translated statement, “and it increases our amazement. In my 25-year career, I have never seen anything like this. Today we hit the jackpot.”

The mountaintop excavation first exposed stone ramparts, but further investigation showed that was just the beginning. The Bronze-Age fortress was clearly designed to withstand time and attacks, and it did so remarkably well—several walls still stood over six feet tall and the mountain’s natural slope helped provide even taller protection.

Investigating the fortress also revealed a three-layered wall, with a central stone layer sandwiched by layers of compacted earth, and several ramparts. “Usually, [ramparts in the Bronze Age] were built of earth and wood,” Potrebica said, “so they fell into disrepair, but here it is different.”

Once past the outer walls, the team found yet another stone wall, which was nearly five feet thick and would have served as a second layer of defense for the area.

When the researchers first arrived at the site, they expected to locate the remains of a newer culture—possibly the La Tene culture, which is associated with the Celts—but found evidence of one much older. While the stone fortress with an intense defensive system was surprising enough, continued exploration of the nearly 10-acre fortress comprising the site suggests that the site was more than a stronghold for soldiers. It may have, in fact, housed an entire community.

Excavations revealed the remains of homes within the walls of the fortress and pottery dating back to the Late Bronze Age, at least as old as 1000 B.C.E. This seems to be evidence of the foundations of Gradina’s organized society—a culture that existed in the Croatian mountaintops much longer and in a much more organized fashion than previously believed.

The coins that launched this whole excavation were a special treasure themselves, linked to the volcanic rocks of the nearby Rupnica and the Tresnjevica quarry. They may have even been minted at the fortress.

Potrebica said that the find forces the research community to rethink the previously accepted social and cultural norms of the Balkans during the Bronze Age—all thanks to some black market Celtic coins.

Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland.