The great thing about a simple game is that you can make it as complex as you like, by looking ahead further and planning. Perhaps spending all your time thinking about decisions from 1 turn is not taxing to your brain. Then, I challenge you to perfectly plan your next 10 turns every turn. There, complicated enough yet? There's such a huge amount of things to think about that I can't keep track of it all. For example, "what is my enemy building" "when will i get attacked" "what tiles are barbarians more likely to be in" "what tiles to use in each city this turn" "what improvements maximize what I want" "will i need extra gold in 20 turns, so should I begin optimizing gold output" "will a granary or monument help me more, and why" "can i survive if I do this" "how can i kill all those units without losing any of mine" "how can I heal my army fastest" "how to move my workers most efficiently" etc. I can spend 5 minutes playing 1 turn in CiV, trying to plan and play perfectly. Every decision, no matter how miniscule, can be analyzed and judged.