"They were probably built in the 14th century, which is pretty old for Stockholm, which was founded in 1252," said John Hedlund, archaeologist at Stockholm Stadsmuseet.
The buildings are made of stone and brick, and were probably houses or storehouses, or a combination of the two.
The archaeologists are digging on Slottsbacken, right next to both the current palace, which is built on the site of the medieval castle. The discovery of at least two cellar rooms took the team completely by surprise.
"We didn't know that there were buildings this close to the castle," he told The Local. "We always thought there was a big gap between the town and the castle."
Indeed the proximity to the castle is puzzling, as most European medieval cities have a large gap between castle and the town. Among other things, this made the castle easier to defend, Hedlund said.
The dig is set to continue on Friday, and Hedlund says there is a possibility that a third cellar will be found.
If the buildings were built in the 1300s, it would make them roughly contemporary with the nearby churches Riddarholmskyrkan and Storkyrkan. A few other surviving buildings in the Old Town are as old, but the new find "probably represents one of the earliest stone buildings in the Old Town," Hedlund said.
The archeologists hope that more clues as to the buildings' histories will be found as the dig continues, but they believe they have already found a channel used to drain sewage from one building to the sea.
The houses were probably abandoned in the first half of the sixteenth century, when King Gustav Vasa strengthened the castle's defences.