Crosmando: 30 years of basically just reskinning the game with minor alterations.
That's a bit unfair. They added something critical in each sequel which changed how the game was played, and not mentioning new AI in every game and new engine fromscratch, and at the same time maintaining the core.
-From CIV to CIV 2 the added the Scenarios and the configuration of the games, the core/complexity of classic Civilization for the next games, firepower and hit points for combat (a new layer for the randomization)
-CIV 2 to CIV 3 they added the cultural borders, national identity, different ways to win, great leaders critical for combat. polishing in the commerce. Now the products are real things and can be traded
-CIV 3 to CIV 4. Policies, economic types, real impact of the religion. Landscape improvements change over time.
-CIV 4 to CIV 5 . From squared tiles to hexagon, and only one unit per tile. Now the warfare works more ike a Panzer General, artillery reaching more than one tile etc. All is less abstracted. minor nations/state cities
-CIV 6. The cities now expand more than a tile. Further estrategy is needed in advace to control the grow of the cities.
There are much more things, they are only a few examples. But every feature changes dramatically how the games work. And every game is worth playing for its own merits. Every fan has its own favourite
Greetings