The highest fortress in Europe?

[LColombo]

I was recently reading about Fort Chaberton. This fortress, armed with eight 149/35 mm guns in eight turrets, was built between 1898 and 1910 on top of Mount Chaberton, at 3,130 m, near the border with France (today, after the territorial changes in 1947, the mount and the fort are in French territory). The fort was involved in the brief fighting along the French-Italian border in June 1940, when it shelled French position but was in turn shelled by French 280 mm mortars, which knocked out six of its eight turrets.
I have never heard of fortresses built above 3,000 meters, at least in Italy (the WWI forts in the north-eastern regions, as far as I know, were all between 0 and about 2,000 m). Are there any forts in France, Switzerland, Austria that were built at a higher altitude, or was this the highest fortress in Europe?