How do I keep long exposures sharp?

I've recently returned from a holiday in Normandy, and I spent a good amount of time playing with my 600d. I took a few different 30sec shots of a village at night, however many of them lack sharpness (which is annoying as I think they are otherwise beautiful) - see the link below for picutre + exif

http://www.flickr.com/photos/sequentialogic/7996902389/lightbox/

I'd love to know what went wrong, I used a tripod (Manfrotto MKC3-P02 - I know it is light, but it was fully retracted and sat on a granite plinth), I used the 2s timer to avoid shaking the camera with the shutter. Maybe it was just bad luck, but I'd love tips to improve!