I don't see why we need to worry about semantics of what a wall is. It makes no difference whatsoever. The only question to answer is how close can the closest defender/s be (includes goal keeper)? if they want to be in a formation of straight line, circle or a triangle is completely irrelevant.
Now given the game is on a non-standard field with modified field markings or field size, I wont be surprised if competition rules also have a modified minimum distance for free kicks. Make sure you check it first.
No matter what that minimum distance is, as mentioned above, if standing on the goal line means they are closer than minimum distance, then they are allowed to stand on the goal line. Its a common sense thing supported by LOTG. Otherwise the opponents will be taking a free kick at an empty/undefended goal.