Summary. There are actually endless miles you can hike up here but our annual tradition is to hike to the overlook above the lake. This ~4 mile hike starts at Castle Lake and goes straight up. Roughly 1.5 miles into the trail, you reach a meadow. From there you can continue to Heart Lake or do what we do, which is veer right and head up and over to the Castle Lake overlook. Dogs are permitted on the trail. There is no water source.
Getting there. From central Mt. Shasta take W. Ream Avenue to W A Barr Road to Ney Springs Road. Take a left on Castle Lake Road. From there it's 7 miles to the parking lot at Castle Lake. Incidentally, this route is also a great, challenging bike route for road cyclists.
Photo above is the cove next to the parking lot. You can swim here or any part of the Lake but access is easiest from this cove. The lake is a high altitude lake so be prepared for a "refreshing" experience. Also, funny side note: the Lake is often a destination for nudists. Yep, you read that right. We have kayaked on this Lake before, only to have a completely nude swimmer pass us by.
The hike. The trail begins just to the left of the Lake if you're facing Castle Lake from the parking lot. It follows the Lake on a briefly flat trail before heading straight up. The trail is challenging in that there's a significant elevation gain over the next mile. There are spots of shade to take a break as needed. In large snow fall years, there can be snow in the shady spots on the trail even in July.
Once you get to a meadow, head to the right and go and over the the steep rocky hill. Here, the trail is unmarked but if you head back towards the Lake, all paths eventually lead to an overlook.
Photo below is from the trail between the meadow and the overlook.
Photo above is from the overlook. We have a family group photo that we have taken every year over the last 15 years right at this spot. Afterwards, continue on the Heart Lake or head back down to Castle Lake.
Additional tip: There are no water sources at Castle Lake so bring your own, including some for your pups. Dogs are allowed on this trail and our two dogs were quite happy to join us.