Originally posted by iamoffline1:

Maybe leave out a 2m piece of wood on the floor of the bridge...?
Not really elegant, but until we get our drawbridges in the vanilla-game, it might help. (If you want, there's a mod with drawbridges called "More Gates", but idk if it's up-to-date)

I'm assuming that the troll will go smashy smashy as soon as he gets to it, but removing that one segment as a failsafe isn't a bad idea.