This house was built before the wall, probably in the mid 13th century. It was incorporated with the earliest wall, enlarged and eventually turned into a defensive tower. The rectangular building has a cellar, and above it a single room divided into two compartments and supported by a barrel vault. The main entrance has been walled up. Traces of it can still be seen on the side outside of the wall. There is an opening above the entrance which may have served as the gate for bringing goods into and out of the building while it was used as a warehouse. During the Middle Ages it was probably also used as a residence.

In the 19th century tar was manufactured in this building, hence the recent name - The Tar Boling House (Tjärkokeriet in Swedish). The tar was used for impregnating rope manufactured in the town. The present-day limestone roof is not consistent with medieval tradition and probably dates from the time when the building was used as a tar manufacture.