den sure kalkun: As stated in the description, this cache was inspired by Terratin. What they didn't know was the cache was hidden exactly halfway between their apartment and mine.
To solve the cache, it was initally a case of going to geotjek.dk and looking at the list of the "hardest" caches to solve. I'd spent a fair amount of time getting this solution to the bottom of that list by putting in hundreds of right guesses. The idea was that it would always be at the end of the list, since it was basically impossible to accidentally get it wrong.
Sadly, some idiot (and I know who) decided to sabotage the solution by putting in wrong guesses (they were guessing coordinates in the southern hemisphere, the same coordinates repeatedly meaning even if they somehow made an honest mistake once, the other 29 times they did it in half an hour could not be thought of as honest).
I was particularly angry about this sabotage. It was pathetic and didn't benefit them in any way. Even after I requested they stop doing it, they carried on.
Luckily, there was a second solution that I knew would appear, given time. You could also search on google within geotjek.dk for the cache GC number. So it was still solvable but not in the way I primarily intended.
Lesson learned. Don't trust fellow geocachers not to be idiots.