Quote from the rules:
Path with a door (x6 + x8 gold cards):
The path behind the door can only be used
by the dwarf of that color. The path for the
other player ends here.
So you can't place new cards behind a door (neither for the troll).
Thanks! That makes sense I guess. The Hungarian rules aren't that clear.
And now that I think about it, the troll shouldn't be a problem, you'd just have to pay him if you'd want to build behind him (just as normally).