Re: A theory on Titans in the Walls (Spoilers)
Woah, crazy thought, but what if this was originally a shifter-built civilisation which regular humans kicked the shifters OUT OF, like stole from them, and so BAR sought to wipe out humanity as vengeance to reclaim the territory for shifters. I mean, yes, shifters have much more of a chance fighting titans than regular humans do, but they can still be swamped by titans and eaten once they're too exhausted to fight on. So even shifters as a society would have to construct walls to stop themselves from being wiped out...
That would make alot of sense especially if you think of it in this line of thought:
Shifter civilization(species numbers in maybe thousands,are in a way immortal and unable to reproduce) encounters human civilization (numbers in millions, can reproduce) a war breaks out becuase of humans natural fear to titans becuase of their size abilities etc
shifters are almost wiped out in war to extinction so they retreat to an area undiscovered by humans and regroup and recouperate, building the current city our characters are in
select titans aka guardians are chosen to speed up building of walls for this haven for shifters
somewhere down the line like you guys said , shifters are displaced out their haven by humans who win by sheer force of numbers
generations pass and titans and the war of the past become fairytale like in status and the humans stay within walls
meanwhile the shifters that survived (willing to bet erens dad is one of the original ones, helped hatch a plot to retake haven and get revenge on humans as well as find a way for titans to reproduce) begin research on how to bolster thier numbers
erens dad integrates himself into current human society gathering intel on humans, has a family and kids (possibly as cover) realizes eren has potential as a hybrid and is the key since he is a hybrid offspring titan/human to making and reproducing titans, which would explain the serum he injects eren with maybe it was a catalyst that reacts with erens blood that his father developed. and would also explain the current happenings of why eren was able to make the other titans do what they did since he would be the source basically.
(sorry for the rambling havent slept in 2 days , but thats what came to me after reading the chapter and these comments)