Mindustry

Mindustry is the entry on this list I almost left out. It is genuinely a factory game that also does tower defense, rather than the other way around. But the wave defense is not optional or incidental. You build resource chains specifically to feed your turret network, and when a wave comes and your conveyor belt logic turns out to have a gap, you feel it immediately. The factory and the defense are the same problem. It runs on almost anything, it is free on PC, and the campaign is enormous. Anyone who has put time into Factorio will recognize the compulsive loop. Tower defense purists should know what they are getting into, but it earns its place.

Mindustry is what you play if Factorio interests you but you want something free, faster to get into, and easier on your CPU. It fuses conveyor-belt logistics with tower defense, so you are routing materials to turrets while simultaneously optimising production chains. The game runs on anything. I tested it on a budget laptop with Intel UHD 620 and it held 60fps without complaint throughout a full campaign mission. The learning curve is steeper than the visual style suggests, but the moment the supply chain clicks and your base defends itself without intervention, you will understand why this one earned its spot.