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Saruman built an army breeding goblins with orcs to make Urukhai and started up forges and training at Isengard as well as improved its defenses. Then to counter Rohan the nearest threat, he built up on pikes and defenses against horses and rallied Dunland men to burn villages and attack people.

Here is my main question. Why didn't Saruman attack the abandoned stronghold of Edoras when he was aware that the main force was at Helm's Deep?

Once Edoras was captured, he could use it as a fort, and then sally forth to block not attack Helm's Deep until they starved out or escaped through the tunnels in which case he could then take and hold Helms Deep.

It could reward Saruman three strongholds and he could begin to start more breeding pits and training halls with forges at all three.

This seems like a better strategy than storming the walls of Helm's Deep which resulted in his eventual defeat.

Answering from memory ... I'll have to check the details later.

Saruman's principle goal at this point was to destroy the Rohirrim as a fighting force. There was nothing to be gained by seizing Edoras, since Théoden had already evacuated it, and even if orcs could be persuaded to live there (they usually prefer caves), the new settlement would not produce soldiers in time for a war that had already started.

As for marching past Helm's Deep, in the book this would have been suicide. Helm's Deep had its own garrison, plus a strong force brought there by Théoden and Éomer. In addition, the forces of the Westfold had been scattered, but not destroyed, at the Battles of the Fords of Isen. They could be regrouped in a few days. Now look at the map.

If Saruman's forces marched east, they would be in hostile territory, cut off from Isengard by a large force of the world's best cavalry. I may not be a wannabe Dark Lord, but this doesn't sound like good strategy to me.

Saruman's actual plan, besieging the main force of the Rohirrim in Helm's Deep (avoiding facing their cavalry in the open) was much better. Even with the relief troops brought by Gandalf and Erkenbrand (the scattered forces from Westfold) Saruman might still have won, if his army could regroup. He probably didn't expect to find a forest of huorns blocking their retreat.

It's harder to explain Saruman's actions in the film. He could have ignored Helm's Deep, given the weakness of the forces there; trying to trap Éomer and overwhelm his riders with superior numbers might have been a better plan. One possibility is that he was determined to kill Théoden, knowing that the king could potentially inspire his people and gather a large army (which he does in The Return of the King). Otherwise, it's just the standard reason: because Peter Jackson.

An assessment of the strategic situation can be found in Unfinished Tales, Part Three, Chapter V “The Battles of the Fords of Isen”.

The conquest of Rohan by Saruman slowed down after Théodred had been slain and thus the first objective had been completed. This delay could be considered a mistake and might have prevented the capture of Edoras. (Note that this aspect partly resembles the famous halt order of Dunkirk in May 1940.)

It was clearly seen in Rohan, when the true accounts of the battles at the Fords were known, that Saruman had given special orders that Théodred should at all costs be slain. At the first battle all his fiercest warriors were engaged in reckless assaults upon Théodred and his guard, disregarding other events of the battle, which might otherwise have resulted in a much more damaging defeat for the Rohirrim. When Théodred was at last slain Saruman’s commander (no doubt under orders) seemed satisfied for the time being, and Saruman made the mistake, fatal as it proved, of not immediately throwing in more forces and proceeding at once to a massive invasion of Westfold; though the valour of Grimbold and Elfhelm contributed to his delay. If the invasion of Westfold had begun five days earlier, there can be little doubt that the reinforcements from Edoras would never have come near Helm’s Deep, but would have been surrounded and overwhelmed in the open plain; if indeed Edoras had not itself been attacked and captured before the arrival of Gandalf.
_{Unfinished Tales, Part Three, Chapter V, 461}

In principle, Saruman’s army could have bypassed the fortress of Helm’s Deep. However, the detour would have exposed the slow Orc infantry in the main body of the army as well as its long line of supply and communication from Isengard to cavalry flank attacks by the Riders of Rohan from Helm’s Deep.

Erkenbrand did not at once himself proceed to the battlefield. All was in confusion. He did not know what forces he could muster in haste; nor could he yet estimate the losses that Théodred’s troops had actually suffered. He judged rightly that invasion was imminent, but that Saruman would not dare to pass on eastward to attack Edoras while the fortress of the Hornburg was unreduced, if it was manned and well stored. (…)
_{Unfinished Tales, Part Three, Chapter V, 466}

Instead, Saruman attacked in full force at Helm’s Deep in order to disable his enemy’s armed forces and leadership.

‘With the help of Shadowfax – and others,’ said Gandalf. ‘I rode fast and far. But here beside the mound I will say this for your comfort: many fell in the battles of the Fords, but fewer than rumour made them. More were scattered than were slain; I gathered together all that I could find. Some men I sent with Grimbold of Westfold to join Erkenbrand. Some I set to make this burial. They have now followed your marshal, Elfhelm. I sent him with many Riders to Edoras. Saruman I knew had despatched his full strength against you, and his servants had turned aside from all other errands and gone to Helm’s Deep: the lands seemed empty of enemies; yet I feared that wolf-riders and plunderers might ride nonetheless to Meduseld, while it was undefended. But now I think you need not fear: you will find your house to welcome your return.’
_{The Lord of the Rings, The Two Towers, Book Three, Chapter VIII}

^{Map taken from Karen Wynn Fonstad The Atlas of Middle-earth, revised edition}

Remarkably, various aspects of Saruman’s campaign are in accordance with the textbook strategy that was customary until the First World War, which is well explained in the famous work On War by Carl von Clausewitz. In particular, concerning the political objective of war, Clausewitz distinguished two broad types:

War can be of two kinds, in the sense that either the objective is to overthrow the enemy – to render him politically helpless or militarily impotent, thus forcing him to sign whatever peace we please; or merely to occupy some of his frontier-districts so that we can annex them or use them for bargaining at the peace negotiation.

Obviously, Saruman chose the first strategic option, which also seems to be in better agreement with his long-term plans.

To attack Edoras would have been useless to Saruman. Theoden and his people had already abandoned the Golden Hall, and had retreated to Helm's Deep. In fact, Aragorn did partially answer this question in the movie.

"They do not come to destroy Rohan's crops or villages, they come to destroy it's people, down to the last child."

Aragorn said this to Theoden while they were inspecting the defenses at Helm's Deep. Saruman's main aim was to subdue the people of Rohan and, if possible, kill Theoden King. Remember, earlier in the series, he had already claimed lordship over Rohan, as mentioned Eomer son of Eomund in both the movie and the book. Saruman got Wormtongue to poison Theoden's mind, allowing his orcs and Uruk-hai to roam at will, killing the people of Rohan.

He did not really need to attack Edoras, as there wasn't anyone there. Thanks to his claimed lordship, he already owned the place in his mind. But he needed to wipe out the people of Rohan, which meant he had to go to Helm's Deep. This is also evidenced by the fact that he used the information Wormtongue gave him after fleeing Edoras to attack the Rohan caravan on the road to Helm's Deep. Given a choice, he would have waylaid Theoden and his people along the way to Helm's Deep with Wargs, and then fallen upon Helm's Deep with it's reduced garrison and wiped them out. But since the attack along the road to Helm's Deep failed, he had to challenge them all at the garrison itself.

This, of course, is following mainly movieverse. However, in the book itself, it is also mentioned that Edoras could have been attacked.

"Where is Eomer? Tell him there is no hope ahead. He should return to Edoras before the wolves of Isengard come there"

Theoden had sat silent, hidden from the man's sight behind his guards; now he urged his horse forward. "Come, stand before me, Ceorl!" he said. "I am here. The last host of the Eorlingas has ridden forth. It will not return without battle."

The man's face lightened with joy and wonder. He drew himself up. Then he knelt, offering his notched sword to the king. "Command me, lord!" he cried. "And pardon me! I thought-"

"You thought I remained in Meduseld bent like an old tree under winter snow. So it was when you rode to war. But a west wind has haken the boughs," said Theoden.

So we can see here that if Theoden had remained in Edoras, Saruman would probably have attacked it. However, since the king had ridden out to war, Saruman's main priority was turned away from Edoras to Helm's Deep.

Another reason, apart from just the fact that the king was in Helm's Deep in that time, could have been that Saruman wanted Helm's Deep for himself first. Edoras was on the plains in the middle of nowhere, so it could wait it's turn. Plus initially, Wormtongue was at Edoras, so the King was essentially his already. However, the garrison at Helm's Deep was strong, and it was almost right on his doorstep. To capture Helm's Deep with or without the King there would have been a massive asset for him.

Specially for that purpose, he created a device that could even bring down the Deeping Wall. It was probably something like gunpowder, but in those times, it couldn't have been easy for him to come up with it, and he must have planned the attack on Helm's Deep for a long time. Between Isengard and Helm's Deep, he could effectively block the Gap of Rohan completely off from anyone. Rohan and Gondor would have been cut off, and it would have been difficult for Rohan to come to Gondor's aid. That way, Saruman would have delt with Rohan, leaving Mordor free to deal with Gondor, starting with Minas Tirith.