Check if you can join a group tour at Roops Fort. If not, this historic building is still worth a photo from the outside.
You're spoiled for sightseeing choice when visiting Susanville. Roop's Fort is an ideal place to begin here, but there are loads of other interesting spots to appreciate. Diamond Mountain Casino, Susanville Ranch Park, and Lassen County Fairgrounds are not far from the historic landmark and will round out your day nicely.