Ludwig von Ysenburg, from a Rhenish noble family, named after his castle built in the Sayn valley around 1100, appears among the heirs of the Lords of Büdingen in 1258. Through skillful politics, he and his successors succeeded in building up a closed territory around the Büdingen Forest, which was significantly increased around 1419 through the acquisition of Dreieich south of the Main and elevated to an imperial count in 1442.
During the reign of Count Ludwig II from 1461 to 1511, the county was finally consolidated and Büdingen was developed into a well-preserved residence and fortress town. His brother Diether rose to become Archbishop of Mainz and founded its university in 1477.
An inheritance contract from 1517 led to the creation of two lines, which were named after their residences Birstein and Ronneburg. The Reformation was introduced under Count Anton (1501-1560). His lively building activities and the pictorial decoration of Büdingen Castle bear witness to the heyday of the Renaissance.
In 1601, the territory was reunited under Count Wolfgang-Ernst (1560-1633), who had turned to the Reformed confession. He began to establish an effective administration and promoted mercantilist mining and metallurgical projects.
However, the turmoil of the Thirty Years' War largely destroyed the reform attempts. As the count's sons joined the Swedish King Gustav Adolf, the county was taken by imperial troops in 1634. Although Büdingen Castle escaped destruction, it was completely plundered.
The Ysenburgers had to go into exile and their land was placed under the administration of the Landgrave of Hesse-Darmstadt. Only the amnesty of the Peace of Westphalia restored the original situation. The old unity of the county was soon lost.
In 1684, was the main division of the still existing lines of Birstein and Büdingen. Three years later, the Büdingen portion was again divided into the lines of Büdingen, Wächtersbach and Meerholz. Despite the fragmentation, these areas achieved great spiritual influence at the beginning of the 18th century.
Under the influence of Pietism, economic and religious motives combined to form a pronounced policy of tolerance. The County of Ysenburg became a "sanctuary of faith" for the oppressed of all kinds: Waldensians and Huguenots, New Anabaptists and Separatists.
Count Ernst Casimir's far-reaching tolerance patent of 1712 also proved to be a magnet for the Herrnhuters expelled from Saxony under Count Zinsendorf, who established the Herrenhaag near Büdingen in 1738. As the "quiet in the land", the so-called "inspired" also found a place to stay until they emigrated to America in 1842.
Political developments in the 18th century with their economic problems prevented Büdingen Castle from being rebuilt in the Baroque style of the time. Its unique character as a manor house of the Staufer era was thus preserved.
In 1806, Karl von Isenburg from the Birstein line, which had been elevated to Imperial Princes in 1744 (and adopted the name form with I), united the sub-counties into a sovereign principality of Isenburg, which only lasted until the Congress of Vienna.
In 1816, the territory was divided up between Hesse-Darmstadt and Hesse-Kassel, and the sovereigns became Lords of the Estates. In 1840, the Büdingen line, in 1865 the Wächtersbach line were elevated to Princes. After the extinction of the Büdingen and Meerholz lines in the 20th century, the lines were reunited under Prince Otto-Friedrich.
In 1943, he and his family moved into the old ancestral castle in Büdingen, which was then carefully restored and opened the castle museum to the public in 1951.