I've received input from different sources on 3 different photos in the past few months that my photos are not quite level. So I may not have a great eye for this.
You may me right that it is sloping to the right, but I don't know how to verify that it actually is, since the there is no clean horizon line or a reference line I can use. I always use a leveler on my camera and tripod. Looking at the photo I see that the castle's vertical lines are slanted to the right, but that may be partly due to the fact the castle is on in the left of the composition and my camera is pointing slightly upwards. Looking at the horizontal castle lines, it actually slopes to the left, which is consistent since the castle is angled away. I can't use the bridge as a reference since it is coming towards us and is arched.
It may be that I need to angle the photo to look straight regardless of what the level is. On this particular photo however, I'm hesitant to do it since it does make the photo smaller, and like the size of it as is.
Hopefully you don't mind my messing with your image, but I pulled it into LR 5 and applied an 'auto' lens correction (didn't have the EXIF information for the lens to profile). If you want to see what LR did, I combined the original and the 'corrected' version. The auto correction may have gone a bit too far the other way; the point is that such things can be easy to fix.