Theme-wise. Note, that while "normal" Titans are grotesque, they look like modern-day Earth humans Overweight, no purpose, mindless consumers. The survivors die in droves, other than the heroes (and a lot of them too) breakdown in the face of the horror, and go mad or turn on each other.
A recurring theme in Attack on Titan is the need to never give up and keep fighting in the face of adversity. If you don't fight, you can't win. Most characters have something that is currently out of their reach:
Eren and Armin want to be free from the walls, but can't because of the Titans. Mikasa wants a peaceful life, but can't be happy unless Eren is safe. Levi and Erwin want a better tomorrow for humanity, but conspiracy factions are holding humanity back. Annie, Reiner and Bertolt want to return home to their village, but can't until they complete their mission. In the past, Hange wanted to learn more about the Titans, but the military refused to support Titan research. Historia wants to live for herself, but other people keep controlling her destiny.
Each character has something they want, but they can never reach their goals if they sit down and do nothing. Humanity as a whole represents complacency. After 100 years, they are no closer to defeating the Titans because the powers within the walls no longer have any interest in doing so. The common people have largely dismissed the existence of Titans prior to Wall Maria's breaching. Humanity cannot win against the Titans because they no longer wish to fight.
The main characters fight, and struggle, and some are even willing to offer their lives while pursuing change. Though the odds seem hopeless, nothing will ever happen unless you are willing to risk sacrificing something.
King (Eren) - The most important piece in the game. If the king is lost, the game is lost. The other pieces will rally around the king, protecting him from capture — often sacrificing themselves to save him. The king can move flexibly, but only one space at a time — any progress the king makes is slow, but steady. Though he is logically more capable than your average pawn, it is a dangerous tactic to use the king offensively. Any movement the king makes must be calculated. Recklessness spells disaster, especially for the king.
Queen (Levi) - The most powerful piece in the game. The queen can move forward, backwards, diagonally, and for as many spaces as it pleases. When it comes to offense, the queen is a powerhouse. It also has great defensive potential, able to swat away any enemy that may come too close to touching the king. In medieval times, the queen was a position of exceptional power. In some cases, the queen actually had more political power and sway than the king himself.
Bishops (Armin & Jean) - Bishops were the religious advisors of the king, and the church was the second greatest influence next to royalty. As such, the bishops start off right next to the king and queen. Bishops can only move diagonally. As a result, they are highly specialized pieces meant for precision strikes that slip between walls. They are also opposites of each other: one travels on black spaces, the other travels on white spaces, symbolizing opposite ideologies of the same faith. Alone, their ability is limited, but working in tandem, they have great potential.
Knights (Erwin & Hange) - Knights were warriors, but were also rich, influential individuals. Highly respected and educated. The knight is a quirky piece. It moves in an L shape. It is one of the least straightforward of the pieces. To be successful, the knight takes very unorthodox, ingenious approaches to take the enemy by surprise. They require forethought to exploit openings that the opponent may have thought were secure. At the very beginning of a game, only a pawn or knight (with its special movement) can make the first move.
Rooks (Mikasa) - Rooks stand on opposite ends of the board, representing the castle — the walls surrounding and protecting the king. The rook is a very easy to understand piece as it can only move in a straight line forward, backwards, left or right. While powerful, a rook needs a clear line of sight to strike an enemy, making it predictable. The rook specializes in physical defense, and is most effective at standing between enemy pieces and a valuable piece, like the king.
Pawns (Krista, Connie, Sasha, Marco, everyone else) - Pawns are simple pieces. They are the foot soldiers; the normals. Individuals who will fight for their cause, and will likely be sacrificed for it. Pawns can only make a single move forward (with the exception of two spaces on their first move), and can only attack diagonally, further limiting their battle ability. It will often be the pawn who will take the first, dangerous step into battle. The most readily at risk. However, should a pawn make it all the way to the opponent's end of the board, they will be transformed into a queen. No life is worthless. The player must consider the weight of what it means to sacrifice a piece, even a seemingly insignificant pawn.