Why did Helms Deep fall within hours?
If you're like me, and God I hope you're not, you're a cynical bastard who smirked and said "they had it coming" as you watched the defence of Helms Deep crumble in the span of one night in Lord of the Rings, the Two Towers.
This has to be one of history's fastest sieges. In our own world attacks on a citadel like Helms Deep could take months or years and were often won not by hordes of people jumping the walls, but with tunnels dug underneath to administer a coup de main or by the starvation of the defenders or like Jericho, through the use of spies.
Let's look at the construction of Helms Deep and find the flaws in it's fortification.
The first thing I note is it's poor location. Fortified structures have always been built to take advantage of the land surrounding them, thus any sane person would have put it on top of the mountain. If the construction didn't allow it should at least have been built several hundred feet up the side of it, yet Helms Deep seems to be built to allow easy access to the enemy.
The main building itself is a concentric construction with two wards and an inner keep. In english this means it has a circular outer wall, or ward, and an inner one, plus a "keep", the fall back location where people go as a last resort. It also has one massive tower which looks entirely useless.
Towers are supposed to be built along the walls to give defenders a good position for providing flanking fire at the enemy, yet the only tower this place has is the huge one in the back which has no real use except perhaps as a lookout location.
Another feature of note is the short wall stretching to the left of Helms Deep. This is where the bulk of the fighting took place in LOTR. The question is, why is this wall even there? It could not be expected to withstand any real assault, but it does give a decent location to harass the enemy. It seems that this wall was built as an outer flanking skirmish line to slow the enemy before the main assault on Helms Deep proper would begin. It's interesting to note, however, that this wall fell about the same time as the rest of Helms Deep. A wall like that shouldn't be expected to last very long, and the fall of Helms Deep proper only minutes later shows how poorly the entire structure was.
It would have been better if Helms Deep were constructed with towers along the walls where the elven archers and other swordsmen could reinforce the main building and fire on the enemy from protected elevated platforms rather than sacrificing them along the skirmish line where they were sure to be routed.
Another thing I note is the walkway leading directly to the front door. What the hell were they thinking? 'Hmmm, let's build a big citadel but leave one nice paved walkway leading up to the biggest door in the whole place, which, incidentally, shall be made of wood which shall be inadequate to withstand more than 10 hits with a relatively small ram.'
Perhaps one of the most important overlooked defence is that of obstructions. There is nothing to slow the enemies approach to the walls of Helms Deep. No moat or trench, no palisade, pointed logs or stakes, no abatis, caltrop, cheval de frise or trou de loup. No entanglements of any sort. These things are essential. They slow the enemy down and even kill many of them. Those that aren't killed or badly wounded are easy targets for the defenders. But none of these were utilized at Helms Deep. The approach couldn't be more hospitable for the enemy, just one big field leading right up to the wall, wide enough to accomidate the full width of the enemy line.
So why was Helms Deep so poorly constructed? One might surmise that Middle Earth fortification science hadn't made it very far yet, but one must note the evolution of such fortifications in the history of the real world. Though Europe is often thought of as the origin of castles and the like, Europeans were in fact many centuries behind the rest of the world.
While Richard the Lionhearted first introduced concentric walls to Europe at Chateau Gaillard in 1197, it was actually based on Byzantine engineering principles that had long confounded the awkward barbarians of Europe but which could have been found in the ancient city of Nineveh about 5000 years earlier. Nineveh was encircled with concentric wards, the inner wall being 120 feet high and 50 feet thick, but more importantly it also had 1500 towers. And that city was founded in 5000 BC and didn't fall until 4388 years later.
Obstructions have been used ever since walls started being used for defence. They're like a pair where and it's hard to think of either being built without the other. Some obstructions such as cheval de frise and trou de loup were products of the Mediaeval ages (obviously given the French names), yet so was the trebuchet and the crossbow which appear in the Lord of the Rings movies.
The abatis was in use during the days of the Roman Empire and towers were in use a few thousand years BC, yet the crossbow wasn't invented until the elenenth century and the trebuchet in the 13th, so don't tell me the people of Middle Earth hadn't figured out how to prepare fortifications that were several milennia older than their weapons.
There is no excuse for Helms Deep to be so poorly constructed, but there is certainly no mystery why it fell so rapidly.
As I said before, they had it coming.