Hi BLS,

To develop a 3D object in the 3D workshop, you should start with a closed polygon and extrude it

If you create a line and extrude it, you will get a surface
(see attached)

Anyway, other good way to create arches in Punch is using empty curved door casements. You can add or not columns between arches
http://www.punchhelpers.com/residential4.html
http://www.punchhelpers.com/imaging6.html
http://www.punchhelpers.com/commercial5.html

Patricia