The world's largest spherical panoramic photograph is a picture of Prague taken from the TV Tower in Žižkov: it has 18.4 gigapixels which were stitched together from 600 individual photographs in October 2009.
The Prague Castle is pretty far from the TV Tower but you can get more details than the picture above. Needless to say, things get much sharper for objects within hundreds of meters from the tower where you can easily read the cars' registration numbers, including one of the black devilish car below:
Shift-click the thumbnails to zoom in.
360cities.net: the Prague browser (arrow keys, +, -)Dresden has a picture with more than 20 gigapixels but it is not spherical.
YouTube demo: focuses on a guy
360cities.net: a blog article
Washington Post, VizWorld, DVICE,
Opposing Views, Wired, SoftPedia