a) the corners are folded along perforated lines and the perforations let light through. Rectify this by inserting additional black paper inside the building. Take care not to cover windows near the corners.

If there is alrge number of windows, consider painting the inside of the walls withblack paint before gluing them together. Take care not to get paint into the joints.
b) The printed windows are usually slightly larger than the actual window frames. Light shines through the edges of the printed window and through the walls around the window frame. Rectify this by applying black tape around the edges of the windows on the insides of the walls. Mask the edges of each window before gluing the walls together.
