Has been free before via
Surviving Mars
The time has come to stake your claim on the Red Planet and build the first functioning human colonies on Mars! There will be challenges to overcome. Execute your strategy and improve your colony's chances of survival while unlocking the mysteries of this alien world.
Also free: Mysteries Resupply Pack DLC
Upvoted even though I own it and all the expansions anyway… Awesome game, played 50 hours or so, I just love city building strategy games.