My space wins on Immortal come typically on continents-like maps after taking over my own continent. I often can't be bothered to invade the other continent(s) as late game wars are annoying busywork and even against the ai landing an army is fraught with peril. In some cases taking over my continent means only conquering a single ai for a total of 15-25 cities. If you want to win peacefully entirely you need to be able at least 10ish decent cities and need some luck. If there is a run-away civ their spaceship will be faster. Ai culture victories will also be faster. If you are lucky the relevant cities (capital/culture cities) are coastal and a few destroyers/transports + 20 marines can take care of it.