IsengardIsengard - Isengard was the northern frontier of Gondor.  Its watchtower, Orthanc, had been manned by soldiers, but when Gondor formed an alliance with Rohan, it abandoned the tower and the Rohirrim protected Gondor's northern flank.  Shortly after, Saruman moved in.

 

"It can thus be readily understood that when Saruman offered to take command of Isengard and repair it and reorder it as part of the defences of the West he was welcomed both by King Fréaláf and by Beren the Steward.  So when Saruman took up his abode in Isengard, and Baren gave him the keys of Orthanc, the Rohirrim returned to their policy of guarding the Fords of Isen, as the most vulnerable point of their western frontier.  There can be little doubt that Saruman made his offer in good faith, or at least with good will towards the defence of the West, so long as he himself remained the chief person in that defence, and the head of its council.  He was wise, and perceived clearly that Isengard with its position and its great strength, natural and by craft, was of utmost importance.  The line of the Isen, between the pincers of Isengard and the Hornburg, was a bulwark against invasion from the East (whether incited and guided by Sauron, or otherwise), either aiming at encircling Gondor or at invading Eriador.  But in the end he turned to evil and became an enemy; and yet the Rohirrim, though they had warnings of his growing malice toward them, continued to put their main strength in the west at the Fords, until Saruman in open war showed them that the Fords were small protection without Isengard and still less against it." ~Unfinished Tales (The Battles of the Fords of Isen)