some of the old light in are build as house and the round part is build on top this room is where the stairs are located to get to the light and it square and narrow on top

[:-/] I am helping a friend who has a preserved lighthouse. I am trying to draw a certain area which needs some repair. How ever the wall are slanted which make the floor lager than the ceiling.
Is there a way to do it. if so how?
Thanks