This is probably the most straightforward legal route to the summit of Castle Peak, and involves some 3rd and possibly 4th class scrambling, depending on your exact route. The approach is largely on trail, with some bushwhacking likely needed closer to the summit towers. The route covers approximately 6.5 miles roundtrip, approximately 2200' of total gain, and takes 5 hours total at a moderate pace.
Overall, the latter half of the climb is characterized by losing, and then just as suddenly finding trail segments. Study your topo beforehand and have a decent idea where you're going, and you should be fine.
The route (click for more info)
See approach route description on the main page.
From the parking area, cross through an opening in the fence and follow the faint, yet obvious trail generally uphill and to the SW. The trail eventually enters a narrow grassy valley in the trees before reaching a partially open area with a large gray dirt slope on the right. Here, turn left and pick up the continuation of the trail as it passes through several scenic open meadows. The summit towers of Castle Peak will be occasionally visible as you gain elevation.
View from the end of the trail
At around 39 45.984 N, 106 49.062 W, the trail appears to fade. This spot overlooks a few small lakes, behind which is a steep semi-forested slope you must gain. The lakes may or may not be dry, depending on the time of year you visit. One option for gaining the slope involves bypassing the lakes on climber's left, then ascending broken trail segments up several more gray dirt slope areas. Another involves cutting to the right, ascending a sort of ridge of gray dirt, bushwhacking up a vegetated slope, and meeting up with a trail segment. Whatever way you choose, you should find yourself in a grassy and willowy valley, with a forested ridge to the left and the obvious summit towers of Castle ahead.
Next, gain the ridge to your left wherever you see fit. There are at least 3 semi-obvious dirt paths leading from the valley to the crest of the ridge, or if these cannot be found, simply bushwhack up. The ridge crest is initially pleasant, but after a steep climb up a meadow, the forest becomes thicker and filled with deadfall. Continue picking your way through this mess. Your goal is to reach the lower edge of a talus field just south of the summit towers, at approximately 39 46.278 N, 106 49.862 W.
Towers 4, 3, 2, and 1
Starting up the talus field
From here, you will be able to see the towers along the summit ridge. There are four main summit towers, which I will number 1-4, from right to left (east to west). Tower 1 is actually two chimneylike towers that look decidedly technical. Tower 2 has purportedly been hand-leveled to be the highest, and looks like a leaning rocky knob. Tower 3 has the appearance of half an upright pop-tart, and looks to be low 5th class and exposed. Tower 4 is at far left. A cliff band mostly blocks access to the ridge crest and towers, but there is an obvious weakness in the cliffs directly below Tower 3. Scramble up the talus to reach the base of this spot.
The summit block, with Matt just below the 3rd class weakness
At the weakness, you have two options: go left, which entails some 4th class climbing, loose rocks and boulders, and at least one awkward move necessitating pulling yourself up. Or, go right on gravel-covered but otherwise very climbable 3rd class ledges. Both options reconvene perhaps 20' above the base of the cliffs, from which the most obvious path upward and to the right will lead you to the ridge crest, immediately east of Tower 3.
From here, drop down the other side of the ridge on a surprisingly wide and unexposed ledge, bypassing a few initial rocky fins to the north. You should see an obvious weakness in the rock, where a large metal cable is hanging down. Scramble up the 3rd class ledges here to the crest of the ridge, turn left, and make the final few easy moves to the unexpectedly-large, flat, grassy summit area. Watch out for the broken glass, relax, and enjoy the views.
Tower 5, from Tower 2
For extra credit, climb "Tower 5", northeast of Tower 2. This much shorter tower can be reached by class 3+ scrambling at worst, but is loose and has daunting exposure, so exercise caution should you try for it.
A helmet would be advised, due to potentially loose rock on the last portion of the route. Climbing gear if attempting the other towers. Trekking poles are useful for the bushwhacking toward the end of the route.
Otherwise, standard hiking gear.