Thinking back about when I moved up in difficulty levels, that's not how it was for me. But I was present here and asked questions, so probably weeded out bad habits during that process. Immortal felt pretty similar to Emperor, and I remember it was so surprising at the time, because I looked at Immortal as an "impossible" challenge. Ofc, that was before I had actually played it. Things generally happen faster on Immortal ofc, the AI has higher bonuses after all and starts with a free worker, but it didn't feel much different than Emperor to me. Deity, however, is a different beast. Bonuses go into overdrive there across the board, and the AIs starting with an extra settler means they grab a pile of land before you know what's what. Looking at above posts again, if a player doesn't use slavery, grab land or improve food early on, Immortal is going to be rough. But that's mainly due to poor habits rather than the game being gruesomely difficult.