Mindustry has a huge update this week adding a ton of new stuff to the 2019 game. The system for controlling units has been hugely upgraded, the campaign completely overhauled, new blocks, turrets, and a logic system for your base, as well as new maps and weather effects for all the old maps. It's a lot, and you can view the whole update on the Steam post.
First released in 2019, Mindustry is a massively well-received hybrid of tower defense, RTS, and factory game. It's quite popular at 97% positive on nearly 6,000 Steam reviews, and fans often cite its consistent large feature updates on top of its already-robust basic feature set as a reason for that value.
I find the new campaign system particularly exciting. The entire campaign now takes place on a big planet, rather than through branching missions. Each sector of the planet is a separate procedural map to visit and fight on—about 260 of them. Meanwhile, the original missions are now special "landing zone" sectors to attack and start new beacheads from. To win you need to finish the landing zones, but the procedural sectors remove the need to replay older missions to gather new resources—some of those sectors even have enemy bases to destroy or capture. Further, and this is kind of wild, "each sector will produce resources in the background even when you play in other sectors." You can then send the resources from one front to another via orbital launch.
The part I'm going to point out as particularly wild is this patch note: "More planets will be added in the future."
Mindustry is only $6 on Steam or pay what you want—which means as cheap as free—on itch.io. (Though the itch.io version doesn't have achievements, seamless multiplayer, or the ease of use that the Steam workshop brings.) I don't know how big the pool of candidates is but I'll go ahead and comfortably say that Mindustry is the best $6 game I've ever bought, even though I think I bought it when it was $3.
Did I mention Mindustry is cross-platform multiplayer? It is. Play it with your friend who only has an iPhone or an Android tablet orwhatever.
It's also a game that you can check out the source code for openly. Free game, but optionally support the creator, and you can browse the entire source code? Feels like the glory days of early 2000s internet to me. Someone please remind the creator that we are all cynical and jaded now, thanks.