Guadosalam is the homeworld of the Guado race which includes Maester Seymour, Jyscal and Tromell Guado. The city is located in an underground, cavern-like structure that has multiple levels (and can be somewhat difficult to navigate). The Farplane can be accessed from there.
The story takes you through Guadosalam and the Farplane only one time. It is the area that the team reaches after they have crossed the Moonflow and just before they reach the Thunder Plains. Check out the strategy guide and walkthrough section for additional details: