Rapperswil Castle dates back around 1200 to 1220 AD, and it was first mentioned in 1229 on occasion of the foundation of the Rüti Abbey. The castle and the fortifications of the former locus Endingen (given by the Einsiedeln Abbey) were built by Count Rudolf II and his son Rudolf III von Rapperswil, when the nobility of Rapperswil moved from Altendorf(Alt-Rapperswil) across the lake to the other side of the so-called Seedamm, maybe to establish their own parish church and to avoid to go the mess, by crossing the lake, in ST.Martin Busskich. As before in the 11th and 12th century AD, the family acted as Vogt of the Einsiedeln Abbey. Sandstonr from the Lützelau island was used to build the castle, the town walls and the city.
The chapel adjoining the ossuary dates back to the time when the parish passed from the Busskirch church to the Rapperswil church and accordingly an inner city cemetery was established. The first chapel was associated to the castle, but the chapel was located outside of its walls and separated by a trench. The preceding building of the Liebfrauenkapelle was built as an ossuary around 1220 to 1253. The charnel house was first mentioned as intra cymeterium ecclesia, meaning church in the cemetery.
The Counts of Rapperswil became extinct in 1283 with the death of the 18-year-old Count Rudolf V, after which emperor Rudolf I acquired their fiefs. The Hersschaft Rapperswil proper passed to the house of Homberg represented by Count Ludwig († April, 27 1289) by first marriage of Countess Elisabeth von Rapperswil. Around 1309 the bailiwick passed to Count Rudolf von Habsburg-Laufenburg († 1315) by second marriage of Countess Elisabeth, the sister of Rudolf V, followed by her son, Count Johann I († 1337 in Grynau) and his son, Johann II († 1380).
In 1350 an attempted coup by the aristocratic opposition (a central person was Count Johann II) in the city of Zürich was forcefully put down, and the town walls of Rapperswil and the castle were destroyed by Rudolg Brun. Eis-Zwei Gesseben, a Carnival festival hold in Rapperswil on Shrove Tuesday, may go back to the siege and destruction of the city of Rapperswil. The battlements and the castle were rebuilt by Albrecht II, Duke of Austria in 1352/54.
After the extinction of the line of Habsburg-Laufenburg in 1442, the castle was given to the citizens of Rapperswil. Ending OLd Zürich war, Rapperswil was controlled by the Swiss Confederation from 1458 to 1798 as a so-called Gemeine Herrschaft, i.e. under control of two cantons of the Old Swiss Conferation and their representative, a Vogt, and Rapperswil castle became an administration site respectively military base and prison.
Over the course of time, the castle fell into disrepair. In 1870 the castle was leased for 99 years from the local authorities by a post-November 1380 UprisingPolish émigré, Count-Wladyslaw Broel Plater (a relative of Emilia Plater, a heroine of the same 1830 Uprising), who had been in Switzerland since 1844. At his own expense he restored the castle, and on 23 October 1870 the Polish National museum was established. Except for two hiatuses (1927 to 1936 and 1952 to 1975), the museum has existed to the present day — an outpost of Polish culture in Switzerland.
In 2008 some Rapperswil residents petitioned local authorities to evict the Polish Museum from its home in the castle, as two historical museum locations (Stadtmuseum and Polish Museum) estimated to be too expensive. The museum was conducting a petition campaign to retain the Museum in the castle, but although the Stadtmuseum (museum of local history was kept respectively renewed at its location at the nearby Breny house at Herrenberg in 2012/13, indeed, the future of the Polish Museum remains unsure.
