If humanity built our city first, then how did they build the wall?

We didn’t build our city first, humanity built their city first, then the Titans, who had no intelligence at all, wandered in and ate everyone. We built our city to protect our humans from the Titans, and we built the walls to keep the Titans out of our city. How did we know of the existance of Titans before we built the walls? Tbh I’m ready to throw the whole humanity underground theory out the window and just say “it’s anime”