In the tactical sense, the AI is a complete joke in civ 1-6 and is not a viable metric for evaluating which side the mechanics favor. From a metagaming perspective, the defender needs to be favored to a degree or attacking is the only strategy that is viable. However, civ games have had the tendency to over-favor the defender. There needs to be some consequence for significantly under-investing in military compared to opposition. Since city defenses were much weaker in civ 4, putting half the into military compared to your opponent meant you could probably be punished...and note that even that game is very defender-favored due to collateral initiative! In civ 5 and 6, the way units can block coupled with the removal of threats like forking make it so that in many situations a defender can win outright while outnumbered 2:1 or 3:1 on equal tech, and that's too far defender favored. At least in civ 6 you have to actually build walls early on if you want them + there is actual incentive to conquer though.