Hi frankiez,
in Germany the general rules are:
- keep minimum distance of 1.5 km from airports, heliports and so one (use Hover app or similar to check)
- keep line of sight
- stay below 100 m altitude
- do not fly over groups of people
- mandatory insurance (not just general public liability, you need a specialized model aircraft insurance)
- if you want to start/land on private ground, you need permission from the owner
- you should also stay away from industrial and military facilities, of course
If you can read German, this is a good summary:
RC-Network Magazin: Modellflug und Luftrecht
Stricter rules apply for commercial use. Local governments may also impose further restrictions.
Regarding Heidelberg, I don't know the exact local rules, but as far as a little googling revealed, the general rules would apply there. So, you could probably do a POI circling of the castle without problems if you manage to keep line of sight, as the castle is surrounded by mostly trees and only a few houses. I wouldn't fly directly over the castle though, as there will always be plenty of people walking around.
Maik