warzone2100 VS Wyrmsun

It's free at wz2100.net and it's tiny, mere megabytes.

Warzone 2100

https://wz2100.net the best free and open-source rts ever. Optimised apple silicon processors

I have always been partial to Warzone 2100. It started off closed source but was opened after the studio closed.

https://wz2100.net/

Here you go: https://github.com/Warzone2100/warzone2100/blob/master/doc/Scripting.md

I haven't dove into the codebase, but I don't think there is a third-party game engine. The engine is part of the warzone codebase: https://github.com/Warzone2100/warzone2100

Also Doom itself and various other ID games are fully open source. In addition there is another game, Warzone2100 was re-released as open source by the developers as well. I'm sure there are more games like these, but this all that comes to the top of my mind.

Warzone 2100 is free and open source. I play it frequently. https://wz2100.net/

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