Combat in Civilization to me will never really be that great because the railroads are fundamentally broken. I remember launching a massive amphibious invasion in Civilization 2 or 3, with 6+ transports loaded with armour and mechanized troops and after quickly capturing a coastal city and digging in, two countries worth of Dragoons which were located on the same continent attacked me the next turn.
Basically every unit in the enemy empire that was available for combat could be instantly transported to the front lines via railroads with no loss of movement, completely devaluing any concept of strategic positioning.
I then focused on destroying the Railroad infrastructure to slow their approach, but with 3 movement the Dragoons could easily and instantly get into combat the turn they were built. Meanwhile my own ships had to sail back to my continent for 3 turns both ways, load up, etcetera before bringing another load of troops.
Conversely, the AI in Civilization has never been able to launch any sort of meaningful amphibious invasion. At best in the older games you'd get a few troops in transports, like 2-3 in a ship that holds 10 and they'd get trounced. That tactic works in the age of sail, not in the age of world wars.
I've heard they've ignored that in recent civilizations by giving any unit the ability to become a transport once it hits the ocean.
Civilization was great fun in my youth, but recently when trying to get into the newer games it no longer appeals to me.