Imrium Castle stands on a rocky crag in the shadow of the Rayesha Mountains, The castle is held by the Imrium Cohort of the Thardic Republic's Gerium Legion and protects a town made wealthy by its role in the salt trade, the source of much of the republic's income.

The Imrium Castle article describes the settlement's history, government, religion, and economics, with special attention to the lucrative salt trade. Detailed descriptions of craftsmen and other locations in the settlement are included, as well as full-color GM and player maps and complete floor plans for the castle, the residence of the district legar, the ruined temple of Agrik, and the Cave of Avathos, where surplus salt is stored and, legends say, an ancient crypt is hidden.