The playhouse is really taking shape now! To create the main window, start cutting from the side wall opposite the chimney. Cut a large square a little over halfway up the wall, but only cut the left, right and top side, leaving you with a flap of cardboard that folds outward from the house.
Use two triangle-shaped supports to keep that flap square to the house by taping one side of the triangles to the outer house wall and the other to the bottom of the flap. This will form the supports and create a nice little table for the outside of the playset. Now cut any other windows your kids would like. You could even include a skylight on the roof!
On one of the long sides of the house, use a ruler to measure about two inches down from the top edge of the roofline. Mark a line across the panel that stops about two inches in from each side. Cut through that line and then cut down the middle, creating two fold-open doors. Cut a hold for the door handles and run a piece of rope through for easy opening and closing.