In a word: killer! We've been hiking here for the past 5 years and have done multiple trails and peaks: Donner Lake Rim Trail, Donner Peak, Mt. Judah, Mt. Lincoln, Pacific Crest Trail, Donner Pass, Northstar, Martis Valley, and Tahoe area like Mt. Tallac, Eagle Lake, Tahoe Rim Trail, Eagle Lake, etc, (check 'em out!) Castle Peak was the toughest. Why? Elevation change, weather change, terrain change, wind change, and temperature change! We're in our late 50s so young whippersnappers might not agree, lol! However, the combination of elevation and conditions made it pretty difficult. There are 2 ways to get to the main trail up to the peak. One goes to the right through a meandering meadow and had good surface. The other main trail is well used and sometimes crowded, and goes to the left up a less interesting very dusty surface. We chose the first and had it almost to ourselves. Because we spent extra time up there and then did a few detour trails on the way down, the main trail down was quickest. Bring lots of water and some energy snacks, solid hiking shoes/boots, extra layers, and motivation! We saw many younger hikers, but the mature folks in their 60s and 70s made us go and were quite motivating. The top is fantastic with a view for miles in good visibility. Go do this and feel like you're literally on top of the world- best hike in years for us and I've been doing this for almost 40 years!! Go get it! read more