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?](https://anime.stackexchange.com/questions/36222/why-are-there-
outlying-towns-cities-on-the-walls-in-shingeki-no-kyoujin)

[![Shignashina
District](https://i.stack.imgur.com/bF0twl.jpg)](https://i.stack.imgur.com/bF0twl.jpg)

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?](https://anime.stackexchange.com/questions/13446/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...

[![enter image description
here](https://i.stack.imgur.com/npbXY.png)](https://i.stack.imgur.com/npbXY.png)
[![enter image description
here](https://i.stack.imgur.com/ohu9B.png)](https://i.stack.imgur.com/ohu9B.png)

And this is where the cannon fired/transformation happened: [![enter image
description
here](https://i.stack.imgur.com/uGC5t.png)](https://i.stack.imgur.com/uGC5t.png)

