Here's a way to create ellipse walls in ADT.
The reason this works is it generates the ellipse as an approximation (a rather good one) and generates it as a segmented polyine.
- Type PELLIPSE and set the value to 1.
- Draw an ellipse with the values needed.
- Right click any wall tool on a tool palette and choose Apply tool properties too... -> Linework. Select the ellipse and you will have a wall that is segmented, but in the shape of an ellipse.