How was helms castle defended in Lord of the Rings?

Answer 1

It was built against a Mountain, between two of its slopes. Meaning it was only approachable from the front.

Helm's Deep also had walls that had withheld many, many battles, The wall was also curved; meaning that archers all along the walls could fire at a target at different angles. this is why the king of Rohan was so confident they would win. It was only after the Orcs created an explosion at the only weak point in the wall, that they were able to compromise its outer defenses.

The only 'civil' way into Helm's Deep was up a long almost sickle shaped slope, the slope had no side guards. Due to the curved wall, archers could fire into the sides of enemies walking up the slope. The attackers could also fall off if they were clumsy, and in the heart of battle, and small mistake like badly placed foot positioning could of made you trip and fall.