This trail starts in the North Cheyenne Cañon Park at the closed section of Gold Camp Road just above Helen Hunt Falls and the intersection of High Drive. For all the information about this park and directions to it, see their website. Take the trail to St Marys Falls.
Elevation Gain ~ 4000
Starting Elevation - 7490'
Length ~ 12 miles out and back
Park at the intersection of the closed Gold Camp Road and High Drive, and start hiking up the closed portion of Gold Camp up to the tunnel. The trail continues up over the top of the collapsed tunnel, and there is a sign for a hiking trail here that heads south towards St Marys Falls. Initially this trail is nice and level walking near a stream, and it gradually picks up to get to the base of the falls. At this point, there will be a sign saying take a left to the base or right to the top of the falls. Take a right and go up a series of switchbacks.
The switchbacks take you above the falls, and you continue on this trail towards the west and Mt Rosa. "After a short level section, the trail climbs steeply up numerous switchbacks, sometimes following old 4WD roads, to the saddle north of Mt. Rosa's summit." - cftbq
Once at the saddle, continue south until you intersect the Frosty Park trail coming up the other side. Join up with this trail and head up the ridge to the summit.
Some good boots and plenty of water. There is water around St Marys Falls, but there isn't anything above that. This is a long climb so be prepared to start early in case of thunderstorms in the afternoons in summertime.