Defenders are very useful to cover stacks of bombard units and overseas invasion stacks. Defenders are essential to all tactics involving funneling (chokepoint or city) to cover the attackers and bombard units. My overseas invasion stacks contain 10-40% defenders dependent on the relative strength, tech and resource availability. Of the defenders I like Infantry, as they have attack of 6 (as cavalry) they can aid in the attack if cavalry got depleted after a proper bombardment. (not taking mechanized into account) The usual ratio of defenders I use is 20-30% in the whole military. p.s. Offense is the best defense but smart offense is better.