Mechanically, Factorio runs on a grid system, and walls are definitely tied pretty firmly to that, with damage to a wall applying only to a specific grid square. Round walls would have to be larger, and thus apply to multiple grid squares, and I'm pretty sure that would end up being a nightmare to ...

Type: Mod
Name: Concrete Walls
Description: Adds an alternate recipe for walls using concrete
License: CC BY-NC 4.0 ->http://creativecommons.org/licenses/by-nc/4.0/
Version: 1.0.0
Release: 2016-03-29
Tested-With-Factorio-Version: 0.12.0
Category: Item
Tags: Walls, Concrete
Download-Url: Concrete ...

Just chiming in with another "thank you." I've burned a whole lot of hours on this, and now I'm getting into modding something for the first time ever, just because this game is so bloody cool.