At least twenty human skeletons were found during excavation work in Vianen, the municipality announced on Monday.
They are probably hundreds of years old.
Archaeologists expect to find more remains.
Alderman Christa Hendriksen said she was "surprised" by the find.
In a statement, she said that some of the skeletons are stacked.
Old nails seem to indicate that it was a wooden box.
No textile remnants, buttons or jewelery have been found near the bones.
"It is clear that this is a historical find. The skeletons date from the Late Middle Ages to a few hundred years ago," says Hendriksen.
It may concern dozens of people who died at the same time, who were buried outside the grounds of Batestein Castle in a mass grave.
Further research should reveal whether this hypothesis is plausible.
Last Friday, the municipality of Vijfheerenlanden announced that nine skeletons had been found.
After the excavation resumed on Monday, it became clear that it was at least twenty.
Hendriksen: "There is no doubt that there is more. The size is not yet known. That is why an even larger area is being investigated."
One of the skeletons found during excavation work in Vianen. (Photo: The Sample)
One of the skeletons found during excavation work in Vianen.
(Photo: The Sample)
Found at the site of the old city castle
The remains were found during excavation work on the city moat.
A section of a filled-in city canal will be reconnected with the rest of the canal.
These activities are now on hold.
Batestein Castle was built around 1370 by order of Gijsbrecht van Beusichem.
Several important noble families, including Van Brederode, inhabited the castle in the three centuries that followed.
The castle fell into ruin after a fire in 1696.