The heart of the city of Utrecht is built on top of a castellum. The potential of the Domplein is to revitalize the significance of the origin of the city. The Castellum wall, at 4 metres underground, acquires a reference in the street and on the square. The dramatic potential of the Domplein is intensified with a clearly recognisable and mysterious dividing line. The marking is just as silent as the quiet archaeological witness underground. The line of light is only interrupted at the Castellum gates. Fragments of smoke come out of the gutter in the metal plates and reveal the presence of the ray of light. The line is even more visible during rain or mist and after darkness has set in.