Multiplayer is finally coming to the excellent Dome Keeper, in a major upcoming update from developer Bippinbits. This is in addition to the developer working on the excellent looking PVKK: Planetenverteidigungskanonenkommandant.
Giving more detail in a Steam post it will include Cooperative Play, Competitive Play, Local Splitscreen and Online Multiplayer. It's even getting a Dome versus Dome mode, which sounds like a lot of fun and a very different experience to the base game.
What's not entirely clear is when it will arrive. All modes are now in the code and working, but they're going through and fixing various bugs as they find them. At some point a big playtest is coming to Steam to help hunt for problems.
Check out the new trailer:

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They also had a developer at the GodotCon 2025 giving a talk on it:

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