When Saruman won passage of the Fords of Isen, he apparently sent his army straight to Helm's Deep. Instead, he should have attacked Edoras, Rohan's capital, and bypassed Helm's Deep-- which even he knew was a very strong fortress. Edoras was not that much further, and supply of the army was not a problem in the Westfold. I think Saruman made a strategic blunder here.