 They established its county seat at what later became Idaho City.The Boise Basin, which contains Idaho City, was one of the nation's richest gold mining districts; gold was discovered in 1862, and more of it was pulled from present-day Boise County than from the entire state of Alaska.At its peak in the mid-1860s, Idaho City was the largest city in the Northwest, and it was this rapid population influx that led to the establishment of the Idaho Territory in 1863.