I play on BI. The AI does far more naval invasions on BI, so you feel less secure leaving your coastal cities almost unguarded.

I don't use any of the BI features like shieldwall or night battles though - night battles look silly IMO and aren't very historical.

I don't use the ALEX engine since I don't like the instant retraining of defeated AI armies that happens when they retreat into a city, it makes me feel that the battle I just won was meaningless.