The remains of the Castle can be visited
Splügen Castle (Ruin)

East of the village of Splügen in the Swiss canton of Graubünden stands the Splügen ruin, a former castle complex. It is the only historical bulwark of its kind in the valley.
History+
Splügen Castle was likely built around 1275 by the Barons of Vaz. Its construction served to prevent the advance of the Barons of Sax-Misox from the Misox Valley over the San Bernardino Pass. They had attempted to expand their dominion by settling Walser people. However, the Walser placed themselves under the protection of the Vaz in 1277, which led to Splügen Castle losing its strategic importance. As early as 1308, the complex is only mentioned as a 'Burgstall' (castle site) in a sales letter, indicating an early decline.
Source: Wikipedia · Licence: CC BY-SA 4.0
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 46.5572, 9.3343