High above the Roofs of Oppenheim sits the skeleton of Landskron Castle. Follow the link for more information (in German or French).
To find the Castle Cache, you have to count the following numbers:
A = Number of Archways in the interior
B = Number of metal bars in the wall opening besides the lower end of the entrance stairway
C = Number of window holes in the southern part of the east wall (multipart windows count as 1)
D = Number of vertical rods of the railing in the southern interior
Then you will find the cache at N 49° 51.3AB E 8° 21.1CD. It is accessible from a walkable area, so you don't need to (and must not!) climb on the walls or enter fenced areas. The box lid is attached to a safety cord, so please don't pull to hard. If there are muggles approaching, you can let it disappear and carry the rest to a better place for logging.
Have fun! :-)