[Jan-Poo]

There's also to consider the fact that the current year in the story is 850, we don't know what it does refers to, but supposedly it's a year count system that has been used for about 800 years. The titans however only appeared some 100 years before.

If this story is set on our Earth and 850 are A.D. then the Titans appeared long before cannons were even invented. That doesn't make much sense, moreover railroads like the ones we see on top of the walls were invented in the industrial Era.

If this story is set on our Earth but in the future, then one would wonder why the A.D. were abandoned and in favor of what. BTW there doesn't seem to be any sign of Christianity left among the humans in the walled country.

We need to consider that the walled country occupies a huge territory, so huge that it can only be in the middle of North America or Asia, if it's really on our Earth. And this huge area not only is completely devoid of any trace of technology, but it looks completely uncontaminated by pollution of any kind.
If Titans were created by eco-terrorist, they fought against something that didn't really have any consistent lasting effect.

PS: I just though of a possible solution. The land they are currently could be the Antarctica, which would explain why there are no signs of technology left and it's consistent with an ecological disaster scenario, moreover it's consistent with the fact that people had to cross the ocean to reach it. However if this is what it is, then Isayama didn't realize that Antarctica would be for the most part submerged by water if all of its glaciers melted, it would become an archipelago. Moreover 100 years do not look enough for such huge tree forest to grow, and if Antarctica became like that before the arrival of titans, it would have been surely colonized by humans