There are multiple ways of bashing this room but this is the way that I did it.
First, this makes for a very tiny room. Cute, but tiny so keep this in mind before you decide to continue with the bash. There is no way to make the room larger without altering the exterior look of the dollhouse, which is not what I want so this is a simple bash keeping as much of the integrity of the dollhouse as possible.
Because of the rooms size, you can now see why I decided to make a faux attic door rather than a real working one. There would just be no space for it. Basically the entire floor of the tiny room would be the attic door, making the room unuseable.
Bashing this tower roof into a room is not as simple as leaving the backside of the roof open. You will need to cut parts. If you don't have power tools, like me, you can use the cutting piece on your rotary tool. I, unfortunately, lost my cutting piece so I had to do this the old fashioned way, with a craft knife/box cutter.
You have to be careful and patient. The plywood is extremely hard. It will take several firm passes with the box cutter to break through it. You can use a metal straightedge to guide your cuts until the box cutter breaks through.
Lay out your tower parts and notice how I cut them in the photos below. You have to cut the bottom tabs off of these pieces as well, if you left your tower base intact, like I did. These tabs are suppose to go into the tower bases center opening but I left that closed so that my tower room would have a floor and the middle third floor room, a ceiling. This means the tabs are no longer needed and if you leave them, your tower room will sit higher up than it's supposed to and not allow you to glue it firmly to the base.
After you have cut your parts, like I did, you have to assemble them as I did in the photos below. If you don't assemble them correctly, you will not be able to fit your curved mansard roof panels later. Remember, you want the curved roof on the front and sides of your room. This is the main architectural feature of this dollhouse so you don't want to alter it.
Below is a photo of the assembled room, positioned on the tower base, as it would look once finished but it is not glued on there yet. This was just a dry fitting so you can have an idea of how it's supposed to look later. I recommend you finish the interior of the room before you glue it in place to the base.
