WARNING: The following contains spoilers for Season 4 of Attack on Titan, now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
As Attack on Titan progresses, the lore behind the Titans continues to grow. 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. While they look similar, there are key differences between the Colossal Titan and the Wall Titans.
What Is the Colossal Titan?
The first episode of Attack on Titan introduces the Colossal Titan, a Titan shifter taller than Wall Maria, which he breaches with a single kick, allowing Pure Titans to flood Shiganshina. These Titans, while tall, are nowhere near the height of the Colossal Titan, who is 60 meters high. Furthermore, these Titans have skin, while the Colossal Titan has exposed muscle.
The Colossal Titan, unlike Pure Titans, is also intelligent. While some Abnormal Titans have demonstrated feats of intelligence, as well as unusual actions, none of them measure up to the Colossal Titan because he's a shifter primarily controlled by Bertholdt, but now Armin is the Colossal Titan. Like other shifters, Bertholdt and Armin can turn into this Titan on command when injured, maintaining their intelligence in this monstrous form.
All Titans also generate excessive amounts of heat, and whenever an Eldian transforms into a Titan, there is a huge burst of energy, but the Colossal Titan's transformation is more versatile, with the holder able to control the amount of energy displaced. For instance, when attacking the walls, Bertholdt's transformation does significantly less damage than his transformation during the Battle of Shiganshina District in Season 3, where he uses this ability like an atomic bomb, destroying countless buildings and killing nearly everyone in the blast radius.
This amount of energy also makes the Colossal Titan generate more heat than other Titans, with the Shifter able to control the amount of steam he emits to keep enemies at bay. The steam is so hot that it can burn a human to death, as is the case for Armin in Season 3, Episode 17, "Hero;" although he barely survived, and the only reason he healed was because the Scouts chose him to eat Bertholdt and become the new Colossal Titan.
While The Colossal Titan is one of the deadliest Titans, this shifter comes with a few downsides. For one, there is the Curse of Ymir, which means any shifter will die 13 years after inheriting their Titan, unlike Pure Titans, who appear to live forever unless they inherit a shifter or have the nape of their neck severed. On top of that, the Colossal Titan sacrifices speed for its size and strength.
What Are the Wall Titans?
Back in Season 2, Episode 1, "Beast Titan," the Survey Corps make an unsettling discovery, finding a Titan living within the walls. It has a similar appearance to the Colossal Titan, having exposed muscle instead of skin, but this Titan is slightly shorter than the Colossal Titan, not reaching the top of the wall.
This Wall Titan remains a mystery until Season 3, Episode 20, "That Day," where audiences learn that King Fritz used millions of Abnormal Titans to make the walls. More information about King Fritz' plan is revealed in Season 4, Episode 5, "Declaration of War." While it's true that he made the walls out of millions of Wall Titans, he never intended to release them.
Furthermore, while these Titans are considered Abnormal Titans, they have yet to show any signs of intelligence or movement. They could be dormant because they were put in the wall by the Founding Titan, so they're waiting for his command. This lack of free will is a key difference between the Wall Titans and the Colossal Titan, who can think for himself and act whenever he pleases.
The Colossal Titan can also freely switch between his human form and his Titan form, while it's assumed the Wall Titans cannot. Without seeing the transformation of these Wall Titans, it's also unclear if they generate as much energy as the Colossal Titan, but it's safe to assume they don't, otherwise Paradis' ecosystem would've been devastated when King Fritz initially made them.
While there are many differences, it appears the Wall Titans can harden their bodies like several other shifters can. Notably, the War Hammer Titan can use a hardening ability to create physical objects. Similarly, millions of Wall Titans used an ability like this to create the walls; however, it's unlikely they could do this without the Founding Titan enabling them since no other Abnormal Titan has demonstrated this skill.
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. Both the Colossal Titan and oversized Pure Titans appear to be slowed down by their larger sizes, but that doesn't make them less dangerous.