yes all cav armies can work. I'm playing as the armenians right now and i basically took over all of russia plus some of the middle east using nothing but horse archers plus the occassional merc (oh and the one unit of cataphracts you get at the start). Only thing you have to worry about is light horse. I've found using armies counsisting of as many HA as you can afford plus 2-4 medium to heavy horse (i was using faction heirs at the start) is pretty much unstoppable, group your HA's in two's or three's activate Cantib circle, then send them off to attack. You'll shortly find the enemy army encircled by 6-12 horse archer circles, the auto skirmish plus your own tender ministrations should be able to keep this up indefinately. It's actually quite a sight, the enemy army will tend to get pull very out of shape as it tries to deal with all these HA units that never quite get in range and because of the Cantib circle they actually stand up to enemy fire better than the enemy seem to be able to respond to theirs.
Eventually it'll all come to an end, most of the time the computer gives up after losing maybe 30-40% of his guys having shot about 1-5% of mine he then starts the long (and arrow filled)trek out of there, if it looks like he'll fight it to the end you just send in the heavy horse once you've used up most of your arrows. Sometimes you get unlucky and lose a unit or two of HA's too a pincer movement but in general it's pretty foolproof, oh yeah keep the heavy horse near the HA's but not too near basically you want his army focussed on the HA's and not on closing with your HH should a unit of HA's receive the attention of some Light Horse or other "interesting" unit just lead the HA's back towards the HH's,basically the HH is just close fire support for any HA's who get in trouble, well that and breaking those final high moral units with charges.
The only real problem with this tactic is it makes seiges a lot more difficult and that i haven't found a real solution to very light horse heavy armies (such as the ones the egyptians have started sending against me after i destroyed the last four stacks of heavy foot that they sent against me).
Oh one last advantage in RTW that Cav have compared to MTW is the increased movement on the strat map, you really can do hit and run tactics on 1500 strong armies with your two units of horse archers as they trundle across the desert.
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