The fort is in Sacramento, CA.
"In 1839 a Swiss immigrant named
John Sutter received a land grant in the Sacramento Valley from the Mexican government. He used the land to create a flourishing agricultural empire and named it New Helvetia (New Switzerland.) This empire established Sacramento’s earliest settlement and the first non-Indian settlement in California’s Central Valley.
"In 1847 Sutter sent aid to the Donner Party, a group of immigrants trapped in a winter storm in the Sierra Nevada Mountains. Word spread and Sutter became known for his hospitality and for providing temporary refuge to travelers. This reputation made New Helvetia the destination for early immigrants to California.
"Less than a decade after they were established, Sutter’s properties were overrun by gold seekers and the fort is all that remains of New Helvetia. It has been restored to its former state based on an 1847 map published in Darmstadt, Germany and is open for tours."
See
http://www.parks.ca.gov/?page_id=485