Unstacking the city is the best change the series ever made — adjacency bonuses make mountain ranges and rivers into planning material, and a badly sited industrial zone is a permanent tax on your empire. The AI is the weak point, but the systems are rich enough to play against yourself.

Fifteen gigabytes, six victory conditions, and a turn that ends when you decide it does. The expansions fix the religion and era systems; the base game is already a hundred hours of planning, and no turn timer is going to help you.