Are there any assets or software that will enable me to easily build 3D roads, such as can be used in a tower defense game?
I use MegaShapes for my roads, has tons of features, runtime lofting, terrain carve or matching etc. The multiple layer types make adding fences or other objects easy, everything stays attached as it should if you change the spline or settings, there is a useful walk loft system for moving objects along the lofts which would be cool for a tower defence game, the cows in the last video and all the otehr objects, trees, jumps etc are done with the walk loft component.
Another tool that is desinged to creat roads is EasyRoads3D Free/Pro