Short answers, pulled from the story.
Isengard is a walled circular enclosure built by men of Númenórean exile in Nan Curunír. The fortress guarded the Fords of Isen from enemy incursions into Calenardhon and stood behind Methedras, the southernmost peak of the Misty Mountains.
Saruman defiled the valley of Isengard with deep pits where he bred powerful warrior Orcs called Uruk-hai. Treebeard led an army of Ents and Huorns to destroy it and flooded the valley while Saruman attacked Rohan at Helm's Deep.
Aragorn visited Orthanc after being crowned King Elessar and found heirlooms of Isildur there including the Elendilmir and a small gold case on a chain that Isildur had used to carry the One Ring.
Tolkien derived the name Isengard from Old English words for iron and court or enclosure which the Rohirrim supposedly gave these names in reality. Both names appear in the poem The Ruin which describes ancient Roman ruins as skilful work and also appear in Maxims II.
Charles A. Huttar describes Isengard as an industrial hell because the ground trembled with treasuries and furnaces lit from beneath while Iron wheels turned endlessly under venomous skies. Saruman's blighted machine-ridden Isengard serves as one example of multiple industrial hells in Tolkien's work.
Peter Jackson based Isengard and Orthanc on Alan Lee's illustrations and Richard Taylor directed the modeling process under his supervision. They used Glenorchy and Mount Aspiring National Park for filming locations where the natural landscape of Glenorchy represented the setting of Isengard.