The shed roof must be trimmed before it can be fitted to the simple cottage. Test fit the roof and bend it to make it sag if you wish. trim the ends and the sides of the roof to suit your cottage. Run a bead of glue (or a glue stick) along the roof edge of the cottage and fit the roof in place.
If you use the cottage as a second story next to the tower on the flat roof mansard house, you will need to set the roof so it matches the edge of the house on one side where it fits against the tower, or you can trim it to fit under the tower.