But despite its decaying state, I thought that the castle deserves its own little treasure. I hope you agree. Besides, as already mentioned above, in Ljubljana and surroundings there are not many places left that justify the hide of a new cache. You have to be satisfied with what the others have left over for you...
Here are some information on this geocaching experience:
The cache is not hidden on the estate, so you do not have to worry about being bothered by the current residents of the castle or their dogs. You will find the micro-/small-sized container in the vicinity of the castle, next to the Gradaščica River.
Logging this cache involves some moderate physical activity and a little bit of courage or mental strength, and maybe, probably, most likely, surely ... (who knows?) some wading or swimming skills 
.
While searching for this cache some stealth is required from your side, since the cache site is visible from the road. So, do expect some walkers, dog owners and/or bikers to pass by and try not to attract their attention! Otherwise, this cache could soon be lost again. Thanks in advance for your caution!
And please, put everything back in the same way as you found it!
Have fun! Vesel geolov!
PS: Cachers who are not only interested in logging a new cache, but who also want to know more about the origin and history of the Bokalce Castle can find some useful background information on Wikipedia (in Slovene and English). Here is the link: http://sl.wikipedia.org/wiki/Grad_Bokalce