@Liz_GoBucks
Edit- They released a new Update on August 18th, where they fixed an issue with the curvature of the roofs. Did that fix your issue?
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Original Post
What are the dimensions of the area under the circular dome roof? For instance, at its widest, how wide is the roof/room going across? Is the circle a very small tower? You should be able to place stairs on the first floor, going to the second, unless there's not enough room to walk or you're intersecting the wall/roof. Try having at least 1 square of space on the second floor around the stairs for your sims to move. You could also try using a ladder and test to see if you can get to the second floor that way. If your sim can get to the second floor with a ladder, the issue might be with your stair placement, and not the roof.
