The fortified mill in Passo di Treia was powered by the water of the Potenza river; it had three millstones (one of which, an original one, was recovered by the city council and is now placed in front of the tower built for the protection of the mill) and was used to grind cereals and to full tissues (the latter an activity which caused a real revolution, since it automated many previously manual activities). The origins of the mill and of its famous tower are very ancient, as they remount to the end of the XIV century. In Treia, then called Montecchio, in just fifty years the city council had built twenty-two mills in an area a few square kilometers wide. The original architectural shape of the structure can be recognized even nowadays. It had a square basis (with sides about 10 meters long) and thick brick walls, was about 18.5 meters high and had two floors; a crawl space used as a depository was subsequently added (the millstones were on the ground floor). The Mill Tower was built to protect the structure from incessant raids and depredations: a document dating from 1191 tells about many incursions the mills in the area had to stand. According to tradition, in 1443 the Tower witnessed the passage of a bombard which Francesco Sforza asked to be sent to Porto Recanati to assault Tolentino. The weapon was transported along the Potenza valley up to the Montecchio (now Treia) and Montemilone (now Pollenza) mills. The Mill Tower belonged to the defensive and control system of the valley and was a strong protection to the bridge, by an important crossroad leading to Treia, Macerata, Urbisaglia and San Severino. In 1965, since the mill was no longer working, the deviation of the Potenza river was filled. The structure is now used as a cultural container for events and photographic shows; when visits by school classes occur, explanatory panels are put in place to show the story of the mill.