In 1302-3 the three Hasenburg brothers, Markward, Heimo and Walter, founded the fortified town of Willisau. The new town was granted the right have a wall and fortifications by the Habsburgs and later the right to hold markets. At the time of its founding it probably had about 150 citizens. Through marriage, in 1367, the Counts of Aarburg inherited the town. However, it still remained a fief under the Austrian Habsburgs. In 1375 Duke Leopold of Austria ordered the poorly fortified city to be burned to prevent its capture by the Guglers, English and French mercenaries who were raiding throughout the region. Just over ten years later, during the Sempach War of 1386, Leopold ordered the town burned again to prevent it falling into Swiss hands.