Titans built the wall to protect the city that no one bothers to actually -blam!-ing attack. Hunters went to go look for stuff outside the wall. Didn't find much, just space pirates, a cultish insect race, militarised pig-apes and deadly robots that now know we exist, shit. Warlocks killed themselves and read books to find out how the big-ass white orb in the sky, that we could easily go inside of and look at cause of the massive gaping wound in it's side, works.