Hi Claude
I don't know, the image is quite small and far away. Possibly florist fern cut into smaller sections then fixed together to create this effect, or maybe another scenic product called "Seafoam". I certainly think its some sort of natural organic plant that's been used to good effect.
Maybe someone will turn up the diorama creators name, get in contact and the mystery will be solved.
I didn't go to Euro that year, but the year after, 2007 and remember seeing your model of the Castle there. Well you couldn't miss it really, as it virtually filled the window area it was located by. It's grown a lot since then of course with even more hours of love and labour devoted to it. Hope it finds a permanent home to be displayed for all to see and admire.
All the best.
Alan