As the achievement description says, you need to build a square-shaped town of any decent size. It’s easiest to do this with smaller towns since you can quickly start a new game to get a new set of tiles.



As shown in my screenshot, it works with a 5×5 layout, it doesn’t even have to be a perfect square. I haven’t tested smaller, but I’d assume any square town larger than 5×5 will also work.