They were holding down the fort.

A Celtic fort destroyed by Roman Emperor Julius Caesar 2,000 years ago has allegedly been uncovered, solving a mystery that has eluded historians for centuries.

As it turns out, the fortification hadn’t vanished; it had just gone underground — archaeologists unearthed a wall and other potential evidence beneath a hospital in Paris, France, not far from Notre Dame.

“This discovery fills an archaeological gap,” Stéphane Deschamps, regional curator of archaeology at Île-de-France Regional Archaeology Service (DRAC), declared in the statement, Livescience reported.

The fort had been used to defend Lutetia, a Celtic pre-Roman settlement on an island in the River Seine in what is now the City of Light.

During the Gallic Wars, from 58 to 50 B.C., the outpost was besieged by Julius Caesar’s forces.

The Dictator for Life reportedly defeated the defenders during the Battle of Lutetia in 52 B.C., but the inhabitants torched the settlement and cut off the bridges, thereby preventing Lutetia from falling into Roman hands.

After going out in a blaze of glory, the fort subsequently became lost to history.

Then, this past February, archaeologists excavating the courtyard of the Hôtel-Dieu — Paris’s oldest working hospital — found a 65-foot charred stone and timber wall that was erected in a distinctly Celtic style.

“The wall’s architecture, with its large blocks and interlocking beam system, immediately led us to believe it was a wall of Gallic tradition,” said the DRAC’s chief curator of heritage Dorothée Chaoui-Derieux in a translated statement. 

At the foot of the now-subterranean battle station was a Gallic spearhead, while nearby burnt seeds and a charred wooden building indicated that the site had gone up in flames.

By carbon dating the wooden elements, the researchers determined that the remnants harked back to between 200 and 10 B.C,” according to Chaoui-Derieux.

When factoring in all these puzzle pieces, the researchers believe thet might’ve discovered “the key to the mystery” of Lutetia, per Deschamps.

If their theory is correct, this could be “one of the most important remains ever discovered for understanding Gallic Lutetia,” said France’s Minister of Culture Catherine Pégard at a conference last month.

She noted that “its function and chronology still need to be verified.”