It's a little bit of work, but it's well worth the effort. A lot of websites, like castles-of-britain.com and castlewales.com have instructions for tourists to find the castle. Using Google Maps, you can get to the right vicinity and then switch to satellite view and scroll around until you find it.
In some areas of the world you cannot zoom in all the way, but in other areas you can get a pretty good image.
This is a good way to research castle layouts if you can't find groundplans.

I have a few more at my website:
http://www.swroble.com/Satellite/SatellitePhotos.html
Stephen