Picked up
mindustry on a whim. It's a tower defense game + a factorio-lite game attached where you build a factory to construct materials to create towers and ammo for them.
The main gameplay loop is that you drop down onto a map with your pod which acts as the core in other tower defense games - lose it and you lose the game. The objective is to survive an unlimited(?) number of increasingly difficult waves while loading resources into the pod. Every 10 waves or so (depends on the map) you can launch your pod into space which ends the map, but adds all collected resources into your central pool. You then use the resources to unlock new buildings, add them to your pod to give you a head start on the next map, or horde them
forever for later.
The maps are all premade, though the landing location and direction enemies come from is semi-random which stops it from getting too repetitive. You unlock new maps by unlocking specific technologies and surviving a set number of waves on the earlier maps. There's 12 (iirc) maps in total, and so far they've all been fairly different from each other with different challenges. The game also has a mapmaker tool and steam workshop support, so you can download fan maps if you get bored of the included official maps.
The factory building is much simpler than factorio, don't expect to spend hours calcuating production ratios and optimizing things to work 2% better. However it's still complicated enough, especially when combined with the increased time pressure to hold my interest. It also has some interesting features like different ammo types for turrets - the basic AA turret loaded with lead shoots high damage single target slugs, but loaded with scrap becomes an AoE flak turret for example. You can also build your own units to fight the enemy, or even unlock buildings that let you transform into different mechs to fight the enemies directly.
It also has multiplayer with co-op, which I've tried and works mostly fine aside from some minor desync issues. There's also a PvP mode of some sort though I haven't tried it so I don't know how it works or if it's any good.
Anyway I think it's a lot of fun. It's also only $6 which is really kind of a steal. If you just want to give it a try I think you can get it for free (or rather "pay whatever you want including $0) from the developer's itch site but it's totally worth the low price to support the developer, plus get steamworks multiplayer and steam workshop support.