I don't know anything about dozers either except that this guy built a tank! Reports say that he even greased the steel plates to prevent people from being able to climb on top of it.
I can't be certain, but it looks like the air intake might have had some protection on it. In that case, he did a mighty fine job of constructing it so it could not be easily stopped.
Has anyone figured out how he got in? I did not see that answered in the article.