According to the interwebs, it's the most important fortress in the canton Grisons. I find it to be, so far, the most beautiful one around as well. It reminds me of the Catalan Romanesque, so maybe I'm not being impartial.
Only the exterior walls and the church tower remain erected, and some of the interior structure can be guessed. The area, which is very well looked after, is reached after a 10 minute walk from the parking lot by the road, past the Santa Maria church.
When I was there last summer, I was the only person around, so I guess it's not very well known or the location is not very convenient for a spontaneous visit if you are not nearby and happen to have some time to spend. I also found a pagoda-like structure, but I couldn't find out what it was for, maybe they celebrate events in there. read more