The secrets of the red bricks

When one walks through the city and looks around, one sees red walls, green walls, gray walls, yellow walls, red walls and again red walls... Perhaps it's just part of the city's style, since it was fashionable to build with red bricks in the 19th century. But maybe the city hides a secret behind its walls? Now we have the chance to find out. Let's walk through the city along the red walls, and maybe then we'll have the chance to reveal the secret.