That diagram is not exactly to scale though - the wall isn't ten miles high. It could just be emphasis on the capital's importance.
The altitude of the human territory increases significantly the closer you get to the centre. The innermost walls seem highest because of sheer height.
When Wall Maria fell, it's stated that 10,000 people or more were devoured. A year later, the government forced 250,000 refugees and soldiers to take part in an operation to reclaim lost territory. All but 100 died. All that death, because of two 11-year old children and a deluded 12-year old.
Reiner: We were just stupid kids that didn't understand anything.
Episode 9 notes that the attack on Trost was so devastating because the Survey Corps were out on an assignment at the time, leaving the most experienced soldiers elsewhere when the proverbial shit hit the fan. The timing couldn't have been worse. The infiltrators knew this, and probably used it to their advantage to maximize the death toll.
The attack in year 845 began just hours after the Survey Corps had returned from a devastating mission outside, using the very same gate.
The Titans in the Walls may or may not be sentient. If they are, that means they've been trapped for over a century and unable to do anything. Little surprise that Pastor Nick is so terrified if that's the case.
It might be slightly better in the sense that being deprived of sunlight, they were probably asleep, or whatever stasis they have instead of sleep... but slightly worse, if that one who briefly woke up somehow changes back into its human form, still inside the wall. No one would know, and it's going to mean bad, bad things the day someone prove dumb or bold enough and lifts up that tarp to check.
Thanks to the revelations from chapter 46, the Running Gag of Reiner expressing the desire to marry Krista whenever she's nice to him gets infinitely more creepy when it turns out he thinks she likes him, but this is part of his overall delusional state concerning himself. Though it's a possibility that he's simply messing with Ymir.
In fact, just... everything concerning Reiner with the revelation that he's been completely insane the whole time.
The whole scene of Eren and Armin talking to Reiner and Bertolt about their past. When confronted about it after The Reveal, Bertolt coldly states that he thought Eren having to watch his mother being eaten was unfortunate.
This seems to be a point of contention in different translations. That line has also been translated as "I felt sorry for you." Still cold, but it implies Bertolt isn't incapable of empathy.
More fun: nakedness is also a sign of purity, especially when it's a kid. The Titans are transformed humans, trapped in a nightmare. They have no idea what they're really doing, and there's a good chance that, if Annie, Reiner, and Bertholdt are any indication, most of them are kids.
In chapter 47, Ymir reveals that she spent 60 years wandering around as a regular (mindless) Titan. It was only five years ago that she was able to wake up and regain her human form. She describes those decades as being trapped in an "endless nightmare", but doesn't recall exactly what she did during that time.....including having devoured Reiner and Bertolt's friend, Berrik. Bertolt states it was "the same" for them as well. If this is the norm for Titans, then that means all Titans were once human. And they all have the potential to wake up, and regain their human minds and forms. That means the only difference between shifters like Eren and regular Titans is that they were able to escape their Lotus-Eater Machine.
In the same chapter Ymir says that "Eren doesn't remember either" which means either a)She is assuming Eren was a titan from outside the walls as well. Or b) Eren killed his dad after he injected him due to a partial or full transformation and reverted back into human form without remembering anything. Possibly c) both. Keep in mind we still don't know where he has been during Eren's training.
In episode 3, Jean says that he is from Trost. So during the Battle of Trost, on top of everything else, he's watching the Titans destroy his home town.
The Wall Cult has a lot of power and influence, and early on, Eren and company complain about what a pain it is when it comes to doing anything to improve defenses on the walls, and how much trouble it was just to get the rail-mounted cannons put up there. The Wall Cult knows about the Titans in the walls. They're desperate to protect the integrity of the walls themselves because, at all costs, they must not be allowed to awaken. The cult was afraid of the noise disturbing them!
The cult leaders stress that the walls "must never be touched". Since Titans require sunlight to function at all, they would most probably not be able to do anything, even if they were to be awoken by a hullabaloo (and it is doubtful that any ruckus, however loud, would be able to rouse them from of a century of induced catatonia in the first place).
How could they be sure? And, possibly worse: the dormancy is all that keeps the Titans in the walls from just breaking out and presumably killing anyone, but if the Wall Cult knows they're people, having the noise wake them when they can't move adds a whole second layer to the I Have No Mouth and I Must Scream aspects of the problem, especially since those Titans are intended to never wake up.
Chapter 50 reveals the real reason why the other Titan Shifters went after Eren. He's a Coordinate, capable of controlling Titans. What about the Beast Titan? And if the abnormal titans have seen this power before, who was the first?
Chapter 51 has several examples.
In-universe example when everyone realizes the true nature of the Titans. Hange and Erwin in particular look horrified as it sinks in that all this time they've been killing and experimenting on people.
When we see the photograph of Connie's parents, it turns out that the Titan Sasha fought at her village was Connie's father.
Combined with what Ymir said about being a titan for 60 years, and the true nature of the titans. How many ways can one become a titan, and what can turn a regular titan into a titan shifter?
It's shown that on the outside walls there are small cities that stick beyond the walls to lure titans there in order to minimize the need of the military along the wall. Then one of those cities was overrun and the entire outer circle was overrun by titans. What happened to those other cities? Best case scenario sees them slowly starving to death, trapped between titans outside the wall, and titans inside the outer wall. Imagine being in one of those cities. Stuck, unable to retreat or stay. Think about being stuck there, starving to death, knowing the only alternative is a gruesome death at the hands of a titan... or a quick plunge off the edge of the wall.
Actually from little we see it looks like the evacuation of Maria went pretty well after the initial attack. Lots of people from the Southern provinces got to safety including the relatively slow boats taking Eren &c. up river. The real damage was done when they promptly all got kicked out again because there wasn't room to settle them, so it's quite likely that the other three wall cities (and settlements in the provinces further North) were evacuated without too much incident (it probably took several days or even weeks for Titans to get all the way around Wall Rose since unless they're actually chasing someone - who wouldn't be going that way - they just amble aimlessly; and the government is shown to have lots of standby plans). Then they all died anyway in the reclamation.
Word of God has said to base the story off his experiences as a victim of bullying. With that some characters and plot points have a more scarier interpretation.
Of course they want women in the military; the last thing the walls need is a bigger population!
But there is an obvious paradox, as the population needs to be maintained in order to supply labour and replace the hordes of soldiers who are continually devoured by Titans.
And , you know, soldiers being scarce enough as they are, without any need of refusing to accept 50% of the population.
Bertolt almost always has an awkward look about him. He's probably not used to infiltrating the ranks of humans as opposed to Reiner and Annie.
In the side story about Ilse Langner, why does the Aberrant react that way towards her?
Because she looks like Ymir, with the same black hair and freckles.
And when it finally does kill her?
It's because she's gotten frustrated with it and lashed out at it. Ymir is usually very calm and snarky. That's how it realized that Ilse wasn't Ymir.
It is implied that the Ape Titan and Titan-Shifters possess an ability to control the behaviour of regular Titans. It is also implied that Ymir "stole" this ability, and may have used it to exert her will upon lesser Titans. The Talking Titan may have initially mistaken Ilse for Ymir, but eventually realised it was not her either due to the difference in their demeanours or the fact that she could not use Ymir's ability, and perhaps even both of these reasons. It is interesting to note that the Titan appeared to be restraining itself from eating Ilse, but since she could not control it as Ymir herself may have been able to, it soon succumbed to instinct and devoured her.
Reiner and Bertolt look younger than they should have, in Reiner's flashback to when he witnessed a Titan devour his friend. It's because it happened prior to them breaching the Wall and infiltrating the human ranks.
Why does the Colossal Titan appear on the exact day that Eren graduates from his training, five years after it's first appearance? Because the Colossal Titan is Bertolt, another of the Trainees graduating and being inducted into the military. It's the first time he's had the freedom to access the Wall without arousing suspicion.
When Connie brings up what his group saw in his Doomed Hometown, Reiner tells him to be quiet, and Ymir jokes around to take his mind off of it, something Reiner asks her to keep doing so he won't worry. Since Reiner is the Team Dad, we can understand why. When The Reveal about him and Bertolt comes around, however, it becomes clear that his main concern was keeping Connie from thinking about the Titan on his house that resembled his mother and even greeted him like a mother would. Note how he only tells Connie to stop talking about it when that part of the story comes up.
Whose idea is it to save the rogue Titan, and who pipes up in favor? Reiner and Annie, respectively. Annie even appears to be addressing Bertolt behind her when she says that the Titan could prove to be an ally.
The Titan forms of all the known Titan Shifters bear at least some resemblance to their human form. The Dancing Titan has Ymir's black hair. The Female Titan shares Annie's Cleopatra Nose. The Armored Titan shares not only Reiner's hairstyle, but even his Heroic Build. The Colossal Titan is able to peek over the wall and Bertolt is 6'3. Finally, the Rogue Titan has the same wild-eyed look as Eren.
Interestingly, the Colossal Titan is the closest of them to having the Uncanny Valley disproportionate appearance typical of Titans. This may hint towards Bertolt's claims that he has "no will of his own" and therefore has less of a sense of self than the others.
Marco looked remarkably relaxed and peaceful for someone bitten in half by a Titan during battle. The other corpses were all twisted into unnatural positions, or had frozen looks of horror on their faces. It's likely he was already dead when the Titan ate him, and Annie murdered him when she stole his gear.
Except titans only eat the living and ignore corpses, see episode 22.
You know that one image of the history of humanity, with the people fleeing the Titans? With what looks like runes? Turns out they're a Genius Bonus for anyone willing to decipher them.
So what does it say? "People fleeing their homes due to the appearance of the Titans. Unable to match the overwhelming force of the Titans, humanity was forced to sail to a new world. At this time, most of humanity had been annihilated, but the majority of this was at the hands of their fellow humans. Those who gained passage aboard the ships were only a small fraction composed of the rich and powerful. The voyage was rough; about half [of the ships?] vanished without reaching the other shore. In the new world, great walls had originally been prepared. This new continent, we shall revere as sacred. Within these walls are the ideals of humanity. Within these walls, let us create a world eternally without strife." Suddenly, so many things either make horrible sense or leave even more horrifying questions in their place.
Why didn't the Armored Titan ever show up at the battle of Trost? Because Reiner decides to follow Jean's order to run to HQ rather than stick to the original plan - just look at Annie and Bert's hesitancy when he bolts. Also, they found out who's inside the Rogue Titan making them to reconsider their decision.
The Walled City seems to have a serious problem with institutionalized classism, with the wealthiest and most talented being given the best and safest places to live, while everyone else is more or less expected to die to maintain the city. The people who originally found the Walls and founded the City inside it were the wealthiest people in the world at the time. When they had to set up a system of government, naturally they would build it around the idea of preserving their own status.
Bertolt makes a big deal about having wanted to join the Military Police Brigade, stating he would probably quit if he failed to rank. However, he ends up being one of several people to enlist in the Recon Corps, a decision he doesn't look too happy about. This was a result of Eren's powers derailing their original plan, which would have required him to be one of those given access to the Interior. Dot Pixis points out that the loss of Wall Rose would result in an even greater purge of the surviving population, and only those with influence or a position within the Military Police would be safe from it. It also would have given him access to Wall Sina without arousing suspicion. But since Reiner joins the Recon Corps in order to remain close to Eren and aid their eventual plan to kidnap him, Bertolt has little choice but to follow him.
If kidnapping Eren was the primary goal of Annie, Reiner, and Bertolt, then why didn't the others help Annie accomplish that mission? Simple. Because their mission also required them to locate the Beast Titan, which had not yet shown himself and if Eren turned out to not be the "Coordinate", still having some of them undercover would help their search efforts.
Reiner did directly help her during the mission. First he told her where he had been told Eren would be in the formation. When that turned out to be disinformation he coaxed Armin into deducing Eren's real position, and relayed it to Annie by swinging into her grasp and carving it into her hand.
More of a 'heh, double meaning' observation, but during the trial, Eren basically tells the court to leave Mikasa out of it (with their panicked suspicions that she might be a titan, too) and that he might be a monster, but she's got nothing to do with it. While he obviously means 'monster' in the literal titan sense, he's probably also acknowledging his (brought up at the trial as well) pitiless killing of Mikasa's kidnappers as a child.
Jean is extremely negative about humanity's chances against the Titans, and is very vocal about wanting to join the Military Police Brigade so that he can live a safe, comfortable life. Now remember that he's from the Trost District, which was safely within the interior until Wall Maria fell. Afterwards, it was flooded with refugees and became the new front line with everyone fully expecting it to be attacked by the Colossal Titan at any point. He went from being completely safe, to living with the constant fear of the Titans and probably witnessed the brutal toll fighting took on the Survey Corps. Little surprise he became desperate to escape that, and return to the safe life he once knew.
Why was Jean automatically trusted and included within the group carrying out the operating within Wall Sina? Because he'd already been cleared of suspicion due to being in the group told that Eren was on the left flank. Since the attack came from the right flank, he couldn't have been the guilty party.
It's a little weird that the regular Titans walk around, some of them with surprisingly well-kept haircuts and groomed facial hair. They don't age, at all, including hair and nail growth. It's also why they don't need to eat; their bodies don't consume resources to grow.
The Female Titan often kills in a highly sadistic manner. It could be that Annie is employing psychological warfare against the Survey Corps. She does not seem interested in killing for its own sake and apparently fights only when attacked. Her mission to find Eren is much more important to her; the flashy kills might serve as encouragement for the Survey Corps to leave her alone. Also hopefully fewer of her classmates would force Annie to do something she would regret; certainly some of them like Sasha and Connie could have their morale broken by hearing of or witnessing such a display.
Ymir comments that she lived 60 years as a normal Titan, with her conscioussness trapped in a state of constant nightmare, unable to control her actions. ZoŽ was right! There ARE human beings inside of Titans and most of them probably aren't malicious and would stop eating humans if they could. Of course, whether or not she can contact them is another story, but now her treatment of Sonny and Bean is completely spot-on, if still futile.
Almost all the people that fight Titans are short and very slim. It is outright stated that a lot of people are unimpressed at Levi being The Ace and being so scrawny and short. But when you fight by propulsing yourself through the air, swinging on ropes and aiming at a very precise weak point with All-Or-Nothing attacks, being small and light is much better than outright physical strength per se. So it is veryJustified.
Also Truth in Television, as any activity that focuses on speed and precision will result in a lean appearance. Gymnasts in particular are on the shorter side, as it's much easier to control their bodies and avoid injury while in close proximity to equipment.
Since Levi is a near-legendary figure, people probably have unrealistic expectations of him. Plus, he is much shorter than most of the cast; this probably surprises people more than his scrawniness.
This theme is very much emphasised by the way quite a few characters, most notably Levi, are depicted to have sparer proportions in the anime than they do in the manga. Many of the cast are not as scrawny as they appear, sporting ripped, athletic figures when de-clothed. As soldiers who utilise gymanastic warfare, they will naturally develop less bulky, but no less powerful, dense lean muscle, and being shorter makes for a more stable centre of gravity; a bonus when keeping one's balance in mid-air.
Why did Reiner and Bertolt cease their attack on the walls during the first invasion? If we observe the scene closely where Reiner busts through Shinganshina's inner wall, we notice that he is within Eren's range of vision as the latter is being transported away on a boat. Their determination to capture Eren hints at them being aware of his power and with the breached areas now open to Titans, what would happen if Eren were to go berserk and direct any invading Titans to attack the duo? They'd be trapped within the walls, surrounded by hostile Titans and the military.
Except at this point they'd probably have no idea that Eren is a titan shifter. It's also hinted that Eren didn't gain his titan shifting abilities until after the first invasion.
Eren tells Levi that he's much more talkative now in the aftermath of his team's death. Levi says he's always talks this this much. Cue a panel showing that they're the only two in a large mess hall. This could imply that yeah, Levi isn't very talkative to Eren. They've only just met. When he's around his now annihilated team, he probably speaks much more, thus accenting how much more alone he's suddenly become.
The other members of his team are much, much more chatty than what we've seen of Levi. It could just be him filling the space left behind.
While this may be too reliant on a specific translation and may not have this double meaning in its original Japanese the fact that the titan spies always refer to their target as the "Coordinate" is rather fitting since the proof that Eren is one of the ones they were looking for is his ability to coordinate packs of lesser titans which in turn means what they are really looking for are "Coordinator" class titans (like the ape) instead of being like a location based set of "Coordinates".
It comes across as being too risky for Annie to kill the captured titans. It wasn't likely that the new experiments were going to reveal anything. However I realize that Eren was suppose to be involved in the experiments. Eren who they already suspected of being the coordinate. In other words had the power to control titans.