The city has approximately 12, inhabitants as of [ update ]. The area of the municipality was already settled in the middle Bronze Age. Fifteen urn burials have been discovered in the municipality. There were late Bronze Age settlements south and west of the modern city. Several Iron Age buildings have been discovered south of town. There is also evidence of a Roman settlement, including a Gallo-Roman mausoleum and a small cache of coins. One or possibly several villas in the area may indicate the existence of a vicus near the town.
The first historic mention of the name dates from to 37 as Delemonte. In , the first mention of the German name Telsperg was recorded. The name is a combination of the Germanic Tello or Dagili with the Latin word mons for mountain. Since the 7th century, the region belonged to the lands of the counts of Alsace. In , it was sold to the Bishop of Basel. At this time it consisted of a fortified village and two castles, which were probably the homes of the Telsberg family.
The city was declared to have municipal rights by Bishop Peter Reich von Reichenstein on 6 January This charter allowed the city some self-governance and allowed it to distribute land, which created favorable conditions for the growth of the town. From this income, as well as the money from a brick factory, two mills and eight farms, which were in use between the 15th and 17th centuries the city was able to meet all of its financial obligations. The old town was nearly square, with two large longitudinal streets that are intersected by three cross streets.
The city was surrounded by city walls, which were strengthened in the southwest, near the Bishop's Castle , during the 14th century. In the north-east corner a large round tower was built in the 13th century.
The latter was bricked up in , when a fire destroyed a large part of the city. The most important public buildings were rebuilt in the 18th century. The Bishop's Castle, which served as a summer residence, was rebuilt in by Pierre Racine from Tramelan.
