These are on the South side of Castle Mountain in an open meadow strewn with boulders. Bob Horan's Bouldering Colorado, Falcon Press, 2008 has a nice picture of the area.
From downtown Estes Park, take highway US 34 for 2 miles, just past Nicky's Resort and Lodge. The meadow is on the north side of the road. Hike north on a faint trail for 100 yards to enter National Park Service land.
Mountain Project's determination of some of the classic, most popular, highest rated routes for Nicky's Boulders: