Not long after the crash, people noticed an area of weirdness. Desiring to keep people out for their own safety, various groups united to build a wall.
The Wall that is currently in place is actually the third attempt; the first two were overtaken by the area’s expansion. Such is the nature of the area that no one knows where, or whether, their remains still exist.
Three governments each claim the Wall as their territory and responsibility. The resulting power struggles mean that it’s actually relatively easy to pass without being noticed, if one is careful.