At the beginning of the 16th century the Duchy of Savoy was at the peak of its power and Ain was inherited by Margaret of Habsburg, the widow of Philibert II, Duke of Savoy. In Brou, she erected a church and a monastery in late-Gothic style. Bourg-en-Bresse became a bishop's see. After Margaret's death Francis I of France, a nephew of the Dukes of Savoy, claimed the Duchy for himself and conquered it in 1536. Following a treaty concluded in 1559 at Savoy, the territory of Ain was restored to the Duke of Savoy who immediately started fortifying it. During the Franco-Savoyard War of 1600–1601 Henri IV of France reconquered the region, though the citadel of Bourg remained impregnable. The Treaty of Lyon of 17 January 1601 finally ended the conflict. Ain now belonged to Burgundy.