Cut out the long walls of the building center. If you like, you can cut out the two open areas between the last columns at the ends of the walls.
Note the reverse folds on the front entryway. Score and fold the piece into the shape in the diagram to make the steps, floor and ceiling of the entry. Glue the side tabs inside the walls, making sure that the floor is level and the wall of the entry lines up with the columns on the outer walls.