The ruins lie just under one kilometre east of the village on a small hill below the former valley road that ran from the Via Mala over the Splügen and San Bernardino Passes.

The elevated entrance with its Gothic archway, with jambs made of tuff, lies at about four metres above the ground on the northern side and leads to the first floor.

The architectural form of Splügen Castle, with its door and window shapes and doorjambs made of tuff, match well to the second half of the 13th century; and it may have been built around 1275.

In that period the valley in the Rheinwald was part of the territory of the County of Schams which, as a fief of the Prince-Bishopric of Chur, was given to the barons of Vaz and, later, the counts of Werdenberg.

This suggests that Splügen Castle, together with the Letzi, had been built around 1275 by the Freiherren of Vaz in order to put a stop to the advances of the Misox clan.

As early as 1308 a purchase deed simply describes it as a castle site (a Burgstall) and a farmstead: "an das Burggstal mit siner gewohnliche hoffraiti".

When the Rheinwald was sold by Jörg of Werdenberg in 1493 to the Milanese army commander, Gian Giacomo Trivulzio, Lord of Sax-Misox, the castle of Splügen was no longer mentioned.