The following verbiage is taken from the National Register of Historic Places Nomination Form in 1974 to describe the history of this fort:
Built in 1854, Roop's Fort is a log structure, 21 x 30 feet, with no foundation and a shake roof. It is quite large as log structures go. The logs are in fair condition as is the roof. It has one floor and a loft. It has received a new shake roof some years ago to copy the old one which had practically vanished. It is located on the original site, and the re-roofing is the only alteration.
Roop's Fort was the first building erected in Lassen County by the White man. It was the first trading post the emigrants encounters after Fort Hall in Idaho on their way west. It was known as Roop House, and later known as Fort Defiance when the boundary dispute (Sagebrush War) was fought between Lassen and Plumas County. Isaac Roop, its builder was the first provisional governor of the territory of Nevada.
The following description is found in California by the Federal Writers' Project:
To the site of Susanville in the spring of 1853, alone and penniless, came Isaac N. Roop on horseback from Shasta. He staked out a claim and built a cabin in July 1854. Here he stored his merchandise and supplies, and to it he brought water from Piute Creek in a half-mile ditch. In its day the cabin even had the honor of serving as capitol of a "territory," which its founders called Nataqua. Nataqua's boundaries henned in 50,000 square miles, from the northeast corner of California to 25 miles south of Lake Tahoe. So far away were California on one hand and the Utah Mormon settlements on the other that the valley's settlers determined to set up their own government. On April 26, 1856, they created Nataqua--and while they were at it, the town of Susanville--meeting at Roop House and electing Isaac Roop recorder. But Nataqua was short-lived. A year later the local men were joining with settlers to the east in a demand for the establishment of the Territory of Nevada. In the same month, August 1857, California asserted jurisdiction when Plumas County created Honey Lake Township. Grumbling, some of the settlers paid their taxes, but not the 40 to 50 pioneers who had endured the early hardships and the Indian fights. In rebellion they had set up a local government in 1858, and the year after helped elect Isaac Roop Provisional Governor of what became, in 1861, Nevada Territory, with Honey Lake Valley included in Roop County.
When California struck to its claims, the stage was set for the Sagebrush War; and Roop House acquired its new name of Fort Defiance. When Probate Judge John S. Ward and Sheriff William Hill Naileigh, of Roop County, refused to refrain from exercising authority, as the Plumas County Court ordered, Sheriff E. H. Pierce, of Plumas County, arrested Naileigh. He sent Deputy Byers to arrest Ward too, but Isaac Roop and seven mounted men blocked the way with shotguns. Forced to give up Naileigh, because snow in the mountains prevented taking him back to Plumas County, Sheriff Pierce crossed the mountains alone. With a posse of 90 men he returned February 13, 1863, to find an armed force of from 75 to 100 fortified in Roop's cabin. Negotiations proved fruitless. On February 15 Pierce and his men occupied and fortified a barn 200 yards away. When one of them, who went out to bring in timber, was fired on and wounded, the battle began; and for five hours both parties blazed away at each other. An armistice followed. Assured by the men in the fort that they would burn down the town around him unless he surrendered, Sheriff Pierce agreed to stop on condition that each party disband and all officers cease functioning. News of the compromise reached a party of reinforcements from Quincy, seat of Plumas County, who were dragging a small cannon over the mountains through the snow. They turned about and dragged it back. The boundary dispute was settled when the California-Nevada Line was run northward from Lake Tahoe, east of Honey Lake Valley; but hard feelings persisted until the California Legislature, on April 1, 1864, created Lassen County with its seat at Susanville.
There is also an official California Historical Marker located a distance from the cabin and it reads: