I'm not totally up to date with the degradation of the sensor, but as far as i know the noise you have is nothing to do with age - if a sensor got damaged you would notice specific dead pixels, stuff like that.
Both of the shots that you show are in tough exposure conditions- the first a long exposure and the second with loads of snow.
Not wanting to say you don't know what you are doing, but I suspect that you used auto metering for both these images? in the castle image the camera has taken an average of the whole scene for the meter reading, but probably the difference between to street and the castle was about 4 or more stops, your image processing software has tried to compensate for this, but this inevitably means noise.
The second shot has all that snow, fooling the camera into thinking it's a brighter day than it was, again when you open in your software you try so get the face correctly exposed and end up adding noise.
So - solutions
in the first shot you could take the same scene either as 2 different exposures and blend them in photoshop (tripod perfect for this) for the castle you could also use manual focus to be sure of being precise. Or you could take 1 image using a neutral density grad inverted from the normal way and darken the foreground.
second shot just needs to be over exposed - unfortunatley there is no way of saying how much - a bit of trial and error in each case