[2] At that time the Counts of Lenzburg were among the most important feudal lords on the Swiss plateau and maintained close connections to the emperor.

Ulrich IV, the last Count of Lenzburg, named Emperor Friedrich I Barbarossa as his personal heir in his will; they had been on the Second Crusade together.

The emperor came to Lenzburg Castle and personally supervised the division of the estate, giving a majority of the lands to his son, the Count palatine Otto of Burgundy.

By way of two neighbouring aristocratic houses (Andechs-Merania and Chalon), in about 1230 Lenzburg castle came by marriage into the possession of the Counts of Kyburg.

They then founded a fortified market settlement at the western base of the castle hill, today's town of Lenzburg.

Rudolph I, Count of Habsburg and later King of the Romans, placed the heir, Anna of Kyburg, under his protection and she later married Eberhard I of Habsburg-Laufenburg.

He was to have married a daughter of King Edward III of England and had the Knights' Hall built for the purpose, but died in 1344 without ever seeing his bride, and the building remained incomplete.

The town of Lenzburg immediately surrendered to the advancing army on 20 April, but the castle for the moment remained untouched by the conflict.

The duties of a Landvogt included collecting taxes, implementing administrative measures, judicial and police tasks and the power of military decree; they were also responsible for the upkeep of the castle.

In 1509–10, extensive work was carried out at the castle, including partial demolition and rebuilding of the unfinished Knights' Hall.

In 1624 Landvogt Joseph Plepp drew the first precise drawings and plans of the castle, which at the time had more the appearance of a fortified farmhouse.

Also, the east bastion had a major disadvantage: rainwater seeped through the adjacent walls and rendered the Landvogt's residence uninhabitable due to persistent damp.

While it flourished, it had 50 students and 12 teachers, with above all sons of prominent manufacturing families in Basel and Alsace receiving their education there.

He had emigrated to San Francisco after the failure of the March Revolution of 1849 and there made a sizeable fortune speculating in land during the California Gold Rush.

Returning to Europe in 1864, in protest against Prussian domination of the German Empire he emigrated once more, this time to Switzerland, and settled in the castle.

To make division of the inheritance possible, the Wedekind family sold the castle in 1893 for 120,000 francs to the American industrialist Augustus Edward Jessup.

Under Jessup's leadership, the castle underwent a comprehensive renovation and by dismantling of the newer construction and military facilities was largely returned to its condition in the Middle Ages.

In addition, he furnished the interiors with expensive furniture and installed modern facilities such as central heating, plumbing and electricity.

Another American industrial magnate, James Ellsworth, who collected medieval art, learnt that Lenzburg Castle contained a table from the period of Friedrich Barbarossa.

The north section is a group of connected buildings, consisting of the upper gatehouse, the remains of the northern keep and the new Landvogt's residence.

The neighbouring Landvogtei at the northeast corner was at the time no longer inhabitable because of penetration of the walls by damp following the construction of the east bastion.

It served as a new office and residence for the Bernese Landvögte after the destruction of the previous building dating to the 14th century in a fire in 1518.

The exception is a small round tower at the southeast corner; built in 1626, it replaced a protruding bay that served as an outhouse.

Today the building houses part of the Aargau Museum, with a permanent exhibit on the lives of the castle owners from the late Middle Ages until the 20th century.

The bastion on the eastern perimeter of the inner bailey was constructed in 1642–46, replacing a curtain wall with battlements, to close the gap between the Palas and the Landvogt's residence and protect the castle from cannon bombardment from the Goffersberg.

During the comprehensive renovation of 1978–86, the last remnant of the embankment was removed and a basement level excavated, which now houses part of the Aargau Museum.

After the death of Ulrich IV, who had ordered it built, the work stopped and the building remained incomplete for almost 200 years.

During the period of Bernese rule, beginning approximately in the early 17th century, the gaol was located on the first floor; it can still be visited today.

To create more storage space for grain, in 1728–29 the tower and the neighbouring Knights' Hall, and the well between them, were joined with an unornamented utility building.

Duke Frederick II of Tyrol-Austria intended to marry the daughter of King Edward III of England at Lenzburg Castle, and thus ordered a suitable Gothic residential building to be erected.