In base game and first expansion, base score of city defense was based on highest land unit (or maybe included ships) you ever created, minus 10 (armies also count to raise this). Then on top of this, add +2 for every city district. There may have been some other factors that add up, but that is gist of it. It was always beneficial to create unit with strongest combat value, at least once or have army with it, to boost defenses. What I noticed that this does count in GS too, but it looks like sometimes city defense of AI cities drops to lower number (!!!). I have no idea why this happens, but have one theory. Maybe score is recalculated based on current troops that AI has. In which case this is very very bad balance change if done intentionally (will make game much easier). Hopefully it is just a bug. Anyone else had experience with this?