Just a quick demonstration of how you can build curved walls/roofs/etc. in the game. I used the Add, Select, Smooth, and Copy/Paste tools to accomplish this. Although this game is in Alpha currently, it is an extremely powerful creative system as it is- and they're still improving it.
Things to keep in mind when making curved surfaces:
1. There needs to be enough voxel thickness to support the curve. If you try to curve a wall that is only 1 or 2 voxels thick you may end up with edges or bumps in the curving where there isn't enough thickness. I've found that 3 or more voxels of thickness usually is sufficient for a smooth curve. For example, if you are smoothing the ground and see poc marks or sharp divits, it's probably because there are empty (air) voxels just under the surface- make sure the ground is at least 3 voxels thick where you are smoothing.
2. In the current build of the game when you are smoothing a Selected area of voxels you must have an empty (air) buffer space around the voxels that you want to smooth, I believe that this is because some smoothing data is currently being stored in those "empty" voxels. This is why in the video, after selecting the voxels I want to smooth, I expand the selection region outward to gain a buffer area that will allow smoothing around the edges. If you do not provide a buffer voxels on the edge of the selection will not smooth correctly, if at all.
3. Use the movement gizmo/mode to place a pasted selection, it's much easier to leave your mouse in one spot that you know won't change as you're building, and just use [tab] and roll the mouse wheel to adjust where the placement will be. You can see this demonstrated as I paste the curved profile in both the horizontal and vertical orientations- note that the mouse cursor doesn't move while I'm placing, and it's not near the area where the selection is being placed.
4. Try again. If one method didn't work, try something else, I've found that if I keep working with the tools I find ways to make them do what I want, or at least come to an acceptable medium, or even come up with a shape I like better.
