The AI tends to only build cavalry when they're attacking, so if you constantly have them on the defencive, you might not see any cavalry.
We're hoping to release our second beta for the next patch (it will either be 1.0.1 beta 2 or 1.1.0 beta 1; we haven't decided) on Monday or Tuesday. If all goes according to plan, we'll have a fail-safe in that release which also allows you to complete kill-quests by exterminating the entire faction.