The Deeping Wall was a large solid stone wall at Helm's Deep.
Built in the founding of Gondor after the Downfall of Numenor by the The Faithful remnant. It was exceedingly thick and strong and has never been breached. It was possibly the most important part of the Fortress of the Glittering Caves stretching from the keep to the end of the opening to the Deep. It also was the most vulnerable after the Dike which is why Aragorn was the one who defended it during the Battle of the Hornburg. It had a culvert which was merely a drainage hole for the water that would accumulate from the steady flow of the Deeping Stream. This weakness was exploited by Saruman who had his Uruks place a fire (possibly some form of explosive known only to Saruman or one of his Order) to create a large enough breach to make the Wall defenseless forcing a withdrawal to the keep.