For experienced players of factory or strategy games, Mindustry starts out really easy. You can just build a wall at a choke point near the enemy spawn, stack up turrets, and go about your business.

At higher levels, it gets more challenging and complicated. You need to produce power and coolant, and you need to use a mix of defenses. And you also need to defend vulnerable sectors against counter-attacks, and build offensive units to attack enemy sectors. And it has a cute little system for programming units in "assembly", although the documentation is almost non-existent.

So even if it seems weirdly easy at first, it might be worth playing at least the first few campaign levels. Overall, I'd say it's my favorite mobile factory game, just edging out Refactory.