A free program for Android, by Anuken.
If you're a fan of the Mindustry sandbox game, check out this classic and pixelated version of it. It functions similarly, but with the added feature of a unique graphic style.
In this game, you can create a systematic supply chain with conveyor belts for feeding turret with ammo, as well as building infrastructures and defending it from opponents trying to destroy them. It has 12 built-in maps with the option to create one of your own through the map editor feature and can be played either over LAN or WAN. You can also enjoy a cross-platform game play using its PC version.
