The idea, as far as Mathilde can tell, is that when someone sneaks up on the Gates from behind, they see unattented bolt throwers pointing at goblins and run right up to pull the trigger, and that's when they run over the wood and gribblies happen.
Goblins aren't stupid, but this is a good example of how they're definitely not smart, either. This is a fun plan, but it's also one with a whole bunch of really obvious flaws that nobody dared to point out to da boss.
I'm gonna have to rephrase that, because I was trying to encompass a range of different items up to about person-sized and didn't think of the obvious conclusion of my phrasing, which is Mathilde running around blatting people in the face with an invisible two-ton boulder.