Isengard was one of the most important towns in Middle Earth with a strategic geographical location. It was also the provisional capital of Rohan. It's importance was comparable to China's Tianjin, Shanghai and other municipalities.
As early as two decades ago, King Théoden, the king of Rohan, handed over the governance of Isengard to his good friend, Saruman the White. Saruman had kept his true nature well hidden. Only when he had Gandalf imprisoned did he reveal his true character. This should only happen more than half a year later.
(ED: Isengard is in fact a Gondorian fortress, and was never under the governance of any Rohirrim king, it was given to Saruman by the then Steward of Gondor, Beren. The author, like in many instances, has taken liberty to mess with the continuity here.)
