How did Titans get inside the wall?
From the books found in Grisha's basem*nt, the Scouts discovered that the Colossal Titans were put within the walls of Maria, Rose, and Sina by King Karl Fritz. One hundred years ago, Karl Fritz was the 145th King of the Eldian Empire.
For reasons unknown, Wall Rose was infiltrated by a foreign assailant. Sometime after his infiltration, he by some means managed to turn the residents of Ragako Village into Titans. These Titans began to wander Wall Rose under his command as the Beast Titan.
They were built by the 145th King of Eldia, Karl Fritz, a century ago using countless Colossus Titans. They stalled the other nations from attacking Paradis, under the false threat that the King would unleash the Titans inside upon the world if they did so.
The walls are made of a number of colossal titans hardening their skin. The only weakest part of the wall is the gate, which is made of some other weaker material for movement. That's why the colossal titan targeted the gate.
Despite being Pure Titans, none of the Wall Titans were seen reverting back into human form after the power of the Titans was sealed.
The two struggle for some time, but Eren is stunned in place upon recognizing a fighting stance used by Annie. In the moment, Annie incapacitates the Attack Titan and captures Eren. As she flees the forest, Mikasa Ackerman and Captain Levi pursue her and successfully immobilize the Female Titan, rescuing Eren.
The only Titan RBA could be 100% certain of being behind the Walls was the Founding Titan. So if someone behind the Walls could turn into a Titan, then those three could not dismiss the possibility that the holder possessed the Founding Titan. They started planning to capture Eren because of that possibility.
Eren possessed the power of three Titans. From his father, Eren inherited the Attack and Founding Titans. After eating Lara Tybur during the Raid on Liberio, he gained the War Hammer Titan as well.
In 845, when Eren was just 10 years old, the "Colossal Titan" suddenly appeared. Large enough to breach the Wall, it allowed other Titans to rampage through the city. One of them ate Carla, Eren's mother.
As the Colossus Titan, Bertolt appeared beyond the south gate of Shiganshina District, breaking the gate apart and breaching Wall Maria in a single kick.
Who is Female Titan?
Annie Leonhart is the Female Titan.
Honestly, they probably waited 5 years because they put off their second attack as long as they could. Judging from the mental state of all 3 of them, being ultimately responsible for the death of 20% of the wall population before you've completed puberty and forced to live among them has got to weigh heavy.
The truth of the walls is that they're composed of gigantic Titans that harden themselves to create three massive walls. These walls were revealed to not be created to protect mankind from the Titans, but to protect the people of Paradis from humans seeking to gain control over the power of the Coordinate.
Titans are typically only active in sunlight. It's theorized that Titans need sunlight for nourishment, since eating people (the Titan's favorite way to terrorize humanity) doesn't appear to provide sustenance.
Anatomical manipulation
The Founding Titan is capable of changing the body composition of any Subject of Ymir. An Eldian king from 600 years ago once used this power to make the Subjects of Ymir immune to a rampant epidemic.
Furthermore, since these Titans can break out of the walls, it's safe to assume they're as strong as the Colossal Titan. They're also likely as slow as the Colossal Titan.
Despite their similarities, the Wall Titans and Colossal Titan are separated by three distinguishing features. Firstly, the Wall Titans are slightly shorter. The Colossal must stand around 60m tall in order to loom over Wall Maria, whereas the Wall Titans can only be as tall as the walls themselves, which stood at 50m.
Abilities. Each of the Wall Titans has the ability to harden not only the top layer of their skin, but much of their body, leaving them deprived of skin and hair.
Only a female human can unlock its power. All the ancient drawings had atleast one titan that looked distinctively feminine (no doubt The Female Titan herself). So if a male inherited this power, it would simply remain dormant. The Female Titan is basically the feminine aspect of Ymir Fritz.
Therefore, if a man were to eat the human wielding this titan, he would possibly retain his masculine features, but his defining characteristic would be to crystallize. Consequently, this means that the name Female Titan is not apt for this titan form.
Why are there only 9 Titan shifters?
I think the reason there are 9 titans is to represent the 9 realms that are connected through Yggdrasil. So, that may be the reason why you won't get more than 9. Inside the story, as it has been said, they have probably tried.
Eren's goal is to eliminate the entire world except for Paradis, but keeping his fellow Paradisans alive is also extremely important to him, especially his close friends.
Armin fires a signal, causing Survey Corps members to emerge and restrain Annie. Seemingly captured and immobile, she still manages to flick her thumb to open a small blade hidden in her ring, which she uses to wound herself and transform into her Female Titan form, killing those restraining her in the process.
Eren's form of the Founding Titan is called Doomsday Titan (終尾の巨人 Shūbi no Kyojin?).
To Hesiod's twelve Titans, the mythographer Apollodorus, adds a thirteenth Titan, Dione, the mother of Aphrodite by Zeus.
Eren's spooky skeletal Titan form was by far the biggest since it was over two hundred and twenty meters tall. Even though Eren only wanted to protect his friends and loved ones he ended up committing horrible acts that eventually led to his death.
In the caption of the post, the account mentions that the author of Attack on Titan, Hajime Isayama, has named Eren's final titan form Shubi no Kyojin, which translated to English means the Final Titan.
As a Titan Shifter, he suffers from the Curse of Ymir, which means that he has 13 years to live after gaining his powers.
It's made clear that this event took place in the year 844. By simple math, one can determine that this would mean Eren and Mikasa are fifteen years old in the year 850 when the majority of the story takes place.
...
When is Eren Yeager's birthday?
Are there Titans inside the Walls?
Along with the revelation that Titans are former Eldians, it's revealed that there are Abnormal Titans living in the walls that protect Paradis, and these Titans have a striking resemblance to the Colossal Titan.
If you're talking about why deos the Collosal titan don't have skin. Then it is probably because of his abilities. His ability is to let off really hot steam capable of burning everything around it.
The titans outside the wall are Eldian Humans that were transformed into Titans via the spinal fluid as punishment for violating Marleyan Laws.
Annie Leonhart is the Female Titan.
1) Founding Titan
This is the most powerful Titan of the lot. If the shifter is of royal blood, they can turn any Eldian into a Titan, control the minds of every Titan and its subsequent shifters, and alter the memories of Eldians. These abilities make it the most powerful Titan in the show.
Because it is impossible for anyone to surpass the Founder, each person who gained the power of the Titans was fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limited their remaining lifespan to only 13 years after first acquiring it.
Attack on Titan fans first met the Female Titan in the form of the mysterious and aloof Annie Leonhart. The Female Titan's powers include agility, speed, and a scream used to attract — but not control — nearby Titans.
I think the reason there are 9 titans is to represent the 9 realms that are connected through Yggdrasil. So, that may be the reason why you won't get more than 9. Inside the story, as it has been said, they have probably tried.
Paradis Island (パラディ島 Paradi-tō?) was a massive island on which the three Walls were located, and Eldia's last territory. It was just off the coast of Marley and is surrounded by the ocean. Marley used the island in the past as a dumping ground for its Eldian criminals, turning them into Pure Titans as punishment.
In 845, when Eren was just 10 years old, the "Colossal Titan" suddenly appeared. Large enough to breach the Wall, it allowed other Titans to rampage through the city. One of them ate Carla, Eren's mother.
