The castle (the Schlössli) and the Altenburg Roman ruins, which are integrated into it, are classified as Swiss heritage site of national significance.

The discovery of several small coin hoards near the ruins may indicate a construction date as early as the beginning of the 4th century AD.

The height of the walls is unknown, but a 7.5 m (25 ft) tall section on south side is still visible, incorporated into the medieval Schlössli.

The Schlössli was built in the ruins of the Roman castle during the late 10th century, probably for an ancestor of the famous Habsburg family.

[2] On 16 November 1414, Emperor Sigismund called the Council of Constance to settle the Western Schism between the three popes (Benedict XIII, Gregory XII, and John XXIII), all of whom claimed legitimacy.