Why can't Titans attack using the canals passing through the walls? I saw this image on this question: Why are there outlying towns/cities on the walls in Shingeki no Kyoujin?



If you look carefully, there is a spot near the bottom where the water canal enters the district Shignashina and then through Wall Maria itself.

Why don't the Titans use this opening to attack the city?
From the question, Can the Titans swim?,
there haven't been any conclusive theories as to how the titans cross the water.


  Titans floating seems misplaced and I think "Connie's Mom" provides evidence that someone probably turned few people living on the islands to titans to wipe out humanity
It's kind of referenced in the anime as "the strongest part of the wall" but not explained...




And this is where the cannon fired/transformation happened: