Information about the actual Barons of Thun is vague and they disappear from sources in the 13th century. There are two versions to how they lost the schlossberg and town. The first indicates that they lost Thun when defeated in battle by the Zähringers in 1190/91. The other version is based upon an arbitration agreement dated 1250 stating that they voluntarily handed over the castle and town to the Zähringers sometime in the middle of the 12th century.
The Knights’ Hall (The Great Hall) would not have been used as accommodation, but as a ceremonial hall for receiving guests, entertaining, and formal functions such as treaty or alliance negotiations, swearing oaths and declaring war.
In 1218, the last Zähringer died and the town and castle passed to the Counts of Kiburg, who allied themselves alternately with the Hapsburgs and the Bernese and called themselves “Neu”-Kiburg. In 1322, they strategically put themselves under the dominion of Berne.
The massive top floor of the Keep (1430-36) comes from the Bernese period. The Keep would not have stood alone, but the residential and administrative quarters of the west wing have been renovated, demolished and/or incorporated into later structures over the centuries.