W a l l s

Fences

It doesn't get much easier than this. Make a vector, loft a Wall, add a texture. If you need more detail:
Planar Textures: 3D Objects & Walls
Walls for Fences

Peaked Roofs

Peaked roofs are an undocumented and unsupported feature in VNS. Sometimes they work, but often they don't. That makes them unreliable in a production environment. By the time you fiddle with vertices to make one work (maybe) you could have modeled the building in a 3D application.

Retaining walls

There are two problems with using a Wall Component for retaining walls: a Wall has no thickness and there's always some space between it and a no-profile ATfx edge. Add to that the irregular vertical 'face' of the ATfx and it doesn't hold up in close views. While you can minimize the irregularity of the ATfx face with high-resolution terrain, high FD, and low Maximum Pixels per Polygon, the render time cost is great and the two problems first mentioned remain. While it is possible to create a retaining wall using a Wall Component, it is not practical and a better solution exists.

I use a 3D Object for retaining walls. They have thickness, are ultimately customizable, hide the irregular ATfx edge, and hold up in close views and from any viewing angle.