The construction of Alzenau Castle between 1395 and 1399 was intended to underline the intention of the Mainz Electors to expand their sovereignty in the Freigericht. The Archbishops Konrad II of Weinsberg and his successor Johann II of Nassau were in charge. The first written evidence of the castle and the name Alzenau can be found in a document dated December 13, 1399. In it, the citizens of the village of Wil-mundsheim, located under the nuwen slosze Altzenahe, are named. The settlement itself is much older. It is first mentioned in the interest register of the Seligenstadt monastery (900/1000 AD).
The Archbishop of Mainz, Johann von Nassau, who as Chancellor was always close to the King, even managed to get King Rupert of the Palatinate (who was a Wittelsbach, a kind of forerunner of the Bavarian rule of the Wittelsbachs from 1816 in Alzenau) to elevate Wilmundsheim to town status on May 13, 1401. However, the expansion of the town was not successful, probably due to warlike events in the 15th century. The reviving settlement was then given the name of the castle. In 1951, the efforts of the Arzenauers were rewarded and the market was given the title of "town" after 550 years.