Isaac Roop built this log building in 1854 in what is now Susanville. It was first called Roop House and served as a stopping place for emigrant wagon trains. Wikipedia calls it the first building built by white settlers in Lassen County.
In 1861 the area was organized as the Nataqua Territory, with its capitol in this building, and it was then incorporated as Roop County, named for Isaac Roop.
In 1863 California and Nevada both claimed the Susanville area, and Nevadans used the fort during what became known as the Sagebrush War. California's claim won out, and the area became part of Lassen County in 1864. The fort is on the National Register of Historic Places (added May 2, 1974) and is California Historical Landmark No. 76.

