This is a route advocated by several denizens of the Conundrum Hot Springs. The local wilderness ranger also endorsed it as the easiest route to either Conundrum or Castle Peak from the west side. Note that east side routes from the Montezuma Basin are far (far) easier. The only advantages to the west side are the hot springs and the opportunity for a more lonesome climb. The day I attempted, I was alone on the mountain.
Follow standard instructions to the Conundrum Creek Trailhead south of Aspen. From Denver, it is a bout a 3.5 hour drive. You can either stick to I 70 as far as Glenwood Springs and the turn back southeast to Aspen, or you can climb two high passes via Leadville to Aspen. It is longer distance by freeway, but the time is about the same. Views are great either direction. Leadville route closed in winter.
From the Conundrum Creek Trailhead, climb to the Conundrum Hot Springs. These are very pleasant, but the area is stressed by their popularity. If you camp there, please bring plenty of sealable plastic bags for packing toilet paper and (best) human waste back out. No dogs or campfires.
From the hot springs, turn east and slip back under the cliff eaves as high as you can stay toward the valley between the northwest ridge of conundrum and the northern flanks of Castleabra. Some of the guys suggested this would take an hour. It took me at age 52 closer to 2 hours. But then I smoked for years and live at sea level in Maryland.
Once, you round the northwest flank of Castleabra, you will see the saddle between Conundrum and Castle pretty much due east of you. There is a bit of a moraine around 11,500 feet with a boggy area above it. Angle north across the bog to a grass-covered high valley between the northwest and west ridges of Conundrum. Once the grass peters out, turn back south to attain the line the west ridge and then follow it up. I had to turn around at abut 12,800 feet because of clouds. Later on, I could see that ridge narrowed considerably toward the top. Locals assured me that the climbing was no harder than class 2, but it did look like there was major exposure on both sides.
Here are some way points in the WGS84 system.
1. N39 00.673 W106 53.319 Just above campsite 13.
2. N39 00.775 W106 53.180
3. N39 00.840 W106 53.095
4. N39 00.897 W106 53.045
5. N39 00.974 W106 52.959
6. N39 00.990 W106 52.900
7. N39 00.950 W106 52.748
8. N39 00.968 W106 52.690 This is where you turn north across boggy area
9. N39 01.103 W106 52.574 This starts narrow valley between NW and W ridges
10. N39 01.132 W106 52.416
11. N39 01.091 W106 52.315 On the W ridge proper
12. N39 01.033 W106 52.296
13. N39 01.016 W106 52.252 Where I stopped because of poor visibility
As far as I made it on the ridge, looking back west.
Raingear. The crossings of Conundrum Creek to reach the hot springs are cold and dangerous. Bridges were out in the summer of 2008. My buddy flashed over to hypothermia from just a few minutes exposure to creek waters after a couple of hours of hiking. Don't be cold before you enter the creek.
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I really wish I had been able to make it. I'd like to try again, but the opportunity might not arise. Kudos to Tom and James of the Hot Springs for recommending the route to me.
This route is not in Roach's book (at least as of the second edition). I had planned on using his 25.5 route over Castleabra, but everyone at the hot springs tried to talk me out of it. Several had done and said that it was very hard and little fun. I would still like to try it, but everyone seemed to agree that the west ridge of conundrum was the better way to go.