Ok there was that final cavalry charge with gandalf but actually the horses would have been shredded by those pikes altough if they overran the first rows...

That was one of the dumbest scenes in the movie. The horses would never have made it down that slope in the first place. They would have rolled, tumbled, legs breaking, Riders flying...and the uruks would have sat there laughing.

in the book eomers there from the start the army is mostly made of dunlenders the uraks are the elites and the huorns (trees) finished off the urak hai gandalfs charge just scared them away.

about 5k rubbish men and 3k uraks approx in book who run v 10k ent like creatures

"the uruk-hai would have killed theoden and aragon when they rode out "

but they didn't, because the men were on horses going at full speed.

In the books gandalf arrived with Erkenbrand and 1000 men on foot, which by themselves would have never stood a chance against the host of isengard. theoden had already been just barely holding out at helms deep with 2000 so 1000 men on foot outside the walls would have still been overwhelmed once the uruks had realized it was only 1000 men they were fighting and not an entire army. the real kicker was the big forest that had arrived behind the isengard forces, they were terrified of the trees, and for good reason. The combined confusion of theodens suicidal charge, gandalfs timely arrival and the sudden presence of a large sentient forest in their to their rear was just too many psychological blows for the uruks to take and they routed and fled into the trees which ultimately took their very brutal revenge on them. The vicotry at helms deep would have not been possible without the hurons. Remove hurons = isengard vicotry. Add hurons = rohan victory