So called semi-underground spaces in Seoul emerged in the 1970s as subterranean air-raid shelters. As the population grew, the spaces transformed into illegal housing units. One of the visible characteristics of the spaces is small eye-level windows that admit little light into the interiors. The project suggests how daylight coming to the interior spaces by day and the artificial light extending to the narrow road by night can be amplified. It took its starting point in a societal situation along with a realistic challenge, and subsequently suggests a solution that seems viable as a very simple intervention of the cubic, sliding window prism.