Helm's deep was tough and I had to redo it twice. What worked for me was to go heavy on cavalry as the archers I built would inevitably get swarmed and get cut to pieces by infantry with scaling ladders, which you can't really kill fast enough to keep any orcs from getting up the walls. If you have 5 or 6 batallions of rohirrim you can let them take the walls, park your heroes into the inner keep, and charge along the walls and mow down the huge hordes of orcs. If you get a blacksmith and upgrade those riders, they can even stand up against the pikeman long enough pulverize them. As an added bonus, they can take out those ballista really quick as well. They did breach the deeping wall and kill all the buildings by the time eomer got his army there, but once he gets there he can easily swarm the enemy camps and crush them if you have built him up a even half decent army.