Why weren't the Titans visible when the Armored Titan broke through Wall Maria? How come in the first episode when the Armored Titan breaks through Wall Maria, there aren't any Titans visible


   given that the wall is made of Titans?


When Eren battles the Female Titan later on


   the Titans in the wall are easily visible.


Was this due to the story having not developed to this point or is there another explanation?
It was noted later that the gate is the weakness of the walls. Why?


   Because there is no Hardened Titan inside, unlike the rest of the walls.


Which is why Colossal Titan targeted it in the first place. Because of this, when Armored Titan charges in, no Titan was shown inside the wall.