Castle Lake is a glacial lake (cirque lake or tarn) located in the Trinity Mountains, in Siskiyou County of northern California. It is west of Mount Shasta City and Mount Shasta peak. The outlet of the lake drains into Castle Lake Creek, and then into Lake Siskiyou reservoir, part of the headwaters of the Sacramento River. Nearly all of the lake's 47 acres (19 ha) are within the Shasta-Trinity National Forest. Trails lead from Castle Lake into the adjacent Castle Crags Wilderness area, and on to Castle Crags State Park, including trails to Little Castle Lake and Heart Lake. Fishing, camping, and hiking are also available at or near the lake.