House of Württemberg
As an Imperial Free City Heilbronn was threatened by the ambitious House of Württemberg. A relationship with the Holy Roman Emperor and a treaty with the Electorate of the Palatinate in effect from 1417 to 1622 strengthened Heilbronn's position and kept the House of Württemberg at bay. The political stability enjoyed by the city during the 15th century enabled it to expand, and many of its historic structures, such as the Kilianskirche (1455–60), trace their origins to that era.