Stratos (aka Presidium) is a science-fiction themed citybuilder game that borrows equally from Majesty, the Fantasy Kingdom Sim and the Impressions Citybuilder series.
This project is currently in a state of suspended development, and no doubt has some significant bugs. Hopefully I'll get around to revisiting in future.
In principle, you should just be able to download one of the standalone app bundles here. Unzip in the directory of your choice, and double-click to play.
The game comes with a full tutorial that should get you up to speed on the essentials, or you can watch the demo video here.