This is how I created an arch using CV curves
Select create CV curve as shown.
Front view ensures that you don't place the curve points all over the place.
Place the points by clicking and create the curve you want. Then press enter.
Now go back to perspective view and extrude the face along the curve, this should then create the curve using the cube as a starting point. I only created half my arch then mirrored it as it seemed easier to get even.
