A hoard of coins has been discovered in Człuchów Castle in northern Poland, dating from the mid-17th century.

A total of 18 coins were hidden in the fortress by its defenders, in a last-ditch attempt to save them from the Swedish army during an invasion.

The treasure was discovered in a defensive gate tower in Człuchów Castle by Maciej Kurdwanowski, one of a team of archaeologists from the University of Warsaw.

There were thought to have been more coins in the hoard originally, but they had been discovered and looted before by a worker in the late 19th or early 20th century. According to the Polish historian Konstanty Kościński, the worker emigrated to America thanks to his find.

The coins had been hidden there during a siege on the castle in 1655-56. This was part of 'The Deluge', a sustained series of attacks by the Swedes on Poland and Lithuania in the mid-17th century.

"Because of their great value in the 17th century, they are rare archaeological finds," research leader Michał Starski from the Institute of Archaeology at the University of Warsaw told the Polish Press Agency.

The coins included large orts with the insignia of Sigismund III Vasa, John Casimier and the Austrian archduke Leopold V. Orts were introduced in the 17th century for foreign trade, but decreased in value rapidly due to American ore flooding the market in the mid-18th century.

"The Człuchów fortress resisted the Swedes for a long time. The siege lasted for several months," said Starski.

But in the winter of 1655-56, the lake surrounding the castle froze over. The Swedish army crossed the icy lake to capture the castle.

"The defenders were unable man the whole, very extensive system of walls."

Elsewhere in the property, archaeologists have discovered the courtyard and cellars of the old castle, built in the 14th century by Teutonic knights. It is not the first time a collection of historic coins has been unearthed in the castle, and it is not known whether there could be more hidden elsewhere in the complex.

The present collection is now on display at the Regional Museum of Człuchów. In late September, the coins will be taken to Warsaw for analysis and conservation, before being returned to Człuchów.