To draw round Walls make sure you are in the Menu Room | Wall.
While drawing, keep CTRL pressed to create an arc.
You can change the radius after placing the arc. Select the round wall, and move the little orange circle upwards or downwards to switch the direction of the rounding.
Creating round murals
To draw a round mural use the free (divider) mural in the menu Room | Mural.
First, you draw the free mural in the desired shape.
Doubleclick on the mural to open the free construction functions. Select the line to form it to a curve or arc. Select the small green circle to change the radius.
Repeat the steps on the other lines to create a rounding.
Take note: Hallows can’t be placed on round and free murals. On the straight part of the free divider mural, it is possible to place a hallow.
If the tiling doesn’t look good on the round part of the mural, you can create extra segmentation points. Doubleclick and add the desired number of points.