Arches?

[Madmike]

so ive been messing with the level editor for about a week now, made some stuff im pretty proud of and started making my level a bit more intricate, but for the life of me i can figure out how to make a custom arch shape. i get the feeling i should be able to do it with the primitives but i cant figure out how to make anything but a flat plane. This has been getting to me for a few days now and i just cant figure it out, any help would be greatly appreciated.