The easiest line up the highest peak in the Castle Group, a multi-day approach from the West Fork Rock Creek drainage to Castle Mountain's south side.
From the West Fork Rock Creek Trailhead outside Red Lodge, a maintained trail leads through Quinnebaugh Meadow and past a series of lakes toward Sundance Pass, gaining views of the Castle Group along the way. Most parties take two days, camping either in the West Rock Creek valley on the northwest side of Sundance Pass or near September Morn Lake on the southeast side, before turning off-trail toward Omega Pass and Castle Mountain's south slopes. The final climb ascends open, moderately steep slopes on the south side of the mountain to the summit plateau; conditions are exposed to weather blowing in from the Pilot Peak direction, so an early start is recommended.