What lays beyond the walls?
The Female Titan arc (57th expedition) DOES NOT take place beyond the walls. It takes place in the area between Wall Maria and Wall Rose, so anything we see there is just a part of the lost territory within the walls. This also includes when we first met Levi and his squad, because the 56th expedition took place from Trost since Maria had already fallen.

MAJOR SPOILERS FROM CHAPTER 85 AND 86 OF THE MANGA


   As it turns out, the walls are on an island called Paradis, with a sizeable amount of land between the walls and the island's coast, but a mainland off the west coast of Paradis is home to humans (the Eldian and Marleyan ethnicities) and is not infested with Titans as Paradis is.