The ruins respectively a chapel built at the castle's former location is situated on the western lake shore of Obersee on a ridge towards the mountain called Etzel in Altendorf.

[1] "Rahprehteswilare" is mentioned for the first time in a document of emperor Otto II, in which goods of the Einsiedeln Abbey in the location of today's Altendorf were confirmed on 14 August 972.

From the previously known, patchy history of the House of Rapperswil only conjectures may done: Certainly it was built well before the year 1200 AD, maybe in 972 or earlier, from which the aforementioned document dates.

A central person was Count Johann II of the House of Habsburg-Laufenburg, who wanted to revenge his father's death at the Battle of Grynau.

The choir of the Chapel of St. John, which is seven-eighths closed, stands on an approximately 70 cm wide foundation of a former round tower of the castle.

It is assumed that the chapel was modelled on the Church of the Holy Sepulchre in Jerusalem to commemorate the crusades and pilgrimages of the House of Rapperswil, because during the excavations a foundation was found which could be the remains of an altar pedestal.

[2] Under the nave of the present chapel of St. John, a wall segment running parallel to the ridge was found with two door sills embedded in it.