The endgame
The Signal
In the original, the space age ended with a ship to Alpha Centauri. Here it ends with a reply.
Listening
The world starts listening one Observatory at a time. When enough nations are running one, a signal comes in five turns later from the outer Oort Cloud, a light-year away. Traced back, it comes from Tau Ceti. Whoever sent it has known about Earth for a long time; a living planet shows up in its own light.
Contact opens a new branch of science. Advances that could not exist before the signal become possible, along with the buildings and units that go with them.
Thirty years
Then the warning comes: something is on its way, and it will make landfall in thirty turns. The game tells you the date. It does not tell you who.
Those thirty turns are the hinge of the endgame. The big defensive project is the Dome, five planetary works anyone may build: the Dome Command Hub, Dome Emitter Array, Dome Kinetic Ring, Dome Power Core and Dome Sensor Net. Finish all five, anywhere in the world, and it counts as a victory in its own right. Against some visitors it is also the difference between being a party to the conversation and being the subject of it.
Winning before they land
A culture or economic win reached while the visitors are still on their way is banked, and the game asks whether you want to stay. Stay to witness the arrival and it is worth a bonus to your score, and your banked victory stands whatever happens next. After landfall, if the world is still yours to play, you can play on toward Alpha Centauri or to the year 2200.