The municipality formed around an abbey founded by St. Trudo, a Frankish nobleman, in the 7th century. Legend has it that as a boy, Trudo was playing while building a small church with some rocks. When a woman scornfully kicked over the rocks she was struck by sudden blindness. Trudo cured her from this blindness after she asked for his forgiveness. He also allegedly was able to spring water and cure other illnesses. After Trudo’s death, the abbey (the later Sint-Truiden Abbey) became the centre of a pilgrimage, which brought wealth to the neighbouring town. The 11th century was particularly prosperous and witnessed an important growth in population. This was the time when abbot Adelardus, reporting to the prince-bishop of Metz, built the abbey’s main church and two additional churches in town: Our Lady (Lievenvrouwenkerk) and Sint-Gangulfus. Under his direction, Sint-Truiden also received an earth wall surmounted by a wooden fence and fortified gates. A proper stone wall, gates and towers, were built in 1129. The economy of this new oppidum city was based on the linen industry and commerce with foreign lands such as England, Champagne, and Germany.
In the 13th century, the fortified town became one of the 23 bonnes villes (main cities) belonging to the Bishopric of Liège. A market hall was built at the site where the current city hall stands, the social life of the city was organized by the various guilds, and a perron was erected on the central square, symbolizing the local government’s authority in political affairs.