The battle at Helms Deep would have worked if only they did a better defence, they knew the orcs were coming, had a few hours or days to prepare, so why not make the people do a moat, traps, or even FIRE arrows and stuff like that..!?
If I knew I had few people and a gigantic army coming, I'd prepare myself for a good fight.. perhaps make fire traps, that may not sound like much, but maybe take out a few hundred fighting orcs.
You forget, King Theodin knew they were in trouble with limited time and resources to defend Helm's Deep. It's why he sent Gandalf to talk to Eomer and bring his Rohirrim back. Obviously the guy left when the King was still under the influence of Wormtongue and Saruman, and Gandalf would be able to straighten things out. Without them coming in on the third day, Helm's Deep would have been lost.
As you'll recall, a moat wouldn't have worked for two reasons: one, they didn't have time to build one, and two, that ramp leading up to the fortress was made of solid stone. It would have taken weeks to break that thing down and build a drawbridge. Second, they already had a "moat" of sorts, but it was mostly just a drainage area, and did nothing to stop the Uruk-hai.
I don't think they had the fuel for fire traps beyond some oil for torches to light the place at night. And remember, they did not have the technology to make cannons or combustible firearms in Middle-Earth. In fact, it was unheard of, and only Saruman managed to figure it out through alchemy. I sincerely doubt he was going to share his new bomb-making recipe with anybody, and I doubt anyone knew about the secret weapon for blowing up the wall aside from him, Wormtongue, and the audience watching the movie.
They had no flank defense, so fighting cornered is all they could do until they did get flanked counter attack assistance from Gandalf and company.
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