The tank that collected the rainwater running off the roofs was situated behind this building. It provided a certain amount of autonomy in the event of a siege. On the right, the chapel, the masterpiece of the castle, was built at the same time as the wall supporting it. The corner turret was probably used as a church tower. It is comprised of two bays: the first was reserved for the bishop's court and the second, topped by a ribbed vault, housed the consecrated altar.
Tourbillon did not remain the main residence of the bishops of Sion for very long. Less than a century after its completion, it was abandoned in favour of the Majorie, bought by the bishop Guichard Tavel in 1373. However, it still played an important military role for many more centuries. Several times during the XIVth century and up until the start of the XVth century, the castle was occupied by inhabitants of Sion who, supported by local lords, were attempting to liberate themselves from the bishop and contest his temporal powers. Tourbillon found itself at the centre of these power struggles and endured them very harshly. It was attacked a number of times and was finally destroyed by fire and demolished at the start of the XVth century.
It was not until the time of William VI of Raron, who was bishop from 1437 to 1451 and an enlightened patron, that the castle was fully rebuilt. The initial structures were retained but everything was redeveloped in the tastes of the time in order to make it a comfortable residence where the bishop lived only during the summer. It stayed this way until the XVIIIth century and did not sustain any more damage, despite the many sieges and internecine struggles that continued in Valais.
On 24 May 1788, a fire ravaged the town of Sion. Fanned by a strong wind, the fire spread to Tourbillon and completely destroyed it. At first the bishop planned to rebuild it but, due to the troubled political situation, this work was never carried out. Thanks to the emergence of an interest in historical heritage during the XIXth century, various conservation campaigns were undertaken in order to prevent the stonework from deteriorating. Tourbillon then became a favourite walk. It was classified as a historic monument in 1907 and now the Tourbillon Castle Foundation actively restores and maintains the site.
