From the Town of Gunnison: Turn north at the center of town on HWY 135. Pass through Almont and continue toward Crested Butte. About 2 miles before reaching the town, turn right onto CR738 which also becomes FR738. This is locally known as the "Pearl Pass Road." The road starts out paved, then changes to graded gravel. It initially passes through a subdivision. Before turning off the highway, you'll have outstanding views of Teocalli Mtn. Passenger cars can make it in the first 4 - 5 miles to a little past the junction where the East River comes in. Just before FR 738 begins to gain elevation, there is a primitive camp area on the right. Passenger cars should be parked here. Higher clearance vehicles and 4WD can continue north. Stay right at the next road intersection in about a mile, continuing on FR 738. The road will drop down briefly to a low water crossing of West Brush Creek. Continue driving another 4.5 miles appx. to the Twin Lakes trailhead parking which turns off the main road to the left.
There are some good, primitive campsites right at the trailhead with some protective trees. You may find other weekend adventurers camped here.
Note: This backpack approach may not only be used for UN13,550 and peaks in that immediate area, but also for the line of summits bordering Conundrum Creek as far north as Hunter Peak. Since Conundrum Creek and hot springs is a very popular destination backpack typically accessed from Aspen and the road to Ashcroft, the access we are suggesting here will avoid the crowds, have a higher starting elevation, provide access to even more summits and offer less competition for campsites.
From the trailhead, hike NW on trail # 402. Initially, you'll have to find a way to cross Middle Brush Creek - possibly using your vehicle or wading. Go north up the hillside some and then turn NW on the trail to the Twin Lakes. It's appx. 2.7 miles to the north end of the northernmost lake at 11,580 ft. with 1,200 feet of gain. The trail crosses into the Maroon-Snowmass Wilderness. Self-registration is required for a permit for overnight visitation. The trail starts out on the east side of the creek but crosses over to the west side a half mile below the lakes. It starts out crossing open terrain, passes through some higher altitude forest and goes through any number of willows as it approaches the lakes. In 2000, when the trail crossed over to the west side of the drainage, it was so difficult to follow in the wet and dense willows, we abandoned efforts to do so and blazed our own path, avoiding some of the willow-bashing. Perhaps it has now been improved.
Trail #402 shows on Forest Service maps as playing out above the Twin Lakes. We actually found usable trail all the way to the saddle WNW of the lakes at 12,500 ft., passing through lush, flowering vegetation on the way up. It was shortly after the saddle that the trail began to play out in the tundra benches above the northern end of West Brush Creek. But it makes little difference. In this location, you can find several campsite opportunities and small tarns for water supply by working your way west from the pass above Twin Lakes. The views and the solitude are outstanding. It is also easy to access "Coffeepot Pass," about a mile NW from the Twin Lakes pass and drop into Conundrum Basin. It's an easy backpack (except for the altitude) over mostly tundra. A short distance down on the north side of Coffeepot Pass, you can pick up Trail #1981 as it drops in from Triangle Pass. This trail drops down to Conundrum Hot Springs almost 2 miles down from Coffeepot Pass, and can serve as your access for Hunter, Keefe and Hilliard Peaks. The trail goes all the way out to CR15 between Aspen and Ashcroft. To avoid possible overcrowded campsite conditions at Conundrum Hot Springs, try camping about a mile up the trail from the springs in the upper basin on some of the tundra benches below the east flank of UN13,216. It may be a 20 - 30 minute walk down to the springs, but you'll enjoy a great deal more privacy and solitude. If packing in from Brush Creek & Twin Lakes, total mileage to this campsite will be about 5 miles with 2,400 feet of elevation gain.
This link from the US Forest Service may provide more information about trail #1981 to Conundrum hot springs: Conundrum Creek Trail #1981 - Hiking Guide
You may be able to find some primitive campsites in the vicinity of the lower Twin Lake. We did not camp at the lakes. We camped well above the lakes on a higher tundra bench area where the trail plays out in the tundra, since we were here to climb multiple summits over several days. In addition, we also camped on a flatter tundra/grassy bench area north of Coffeepot Pass by about 3/4 mile, well off the trail and east of the UN13,216 summit at an elevation of around 12,200 ft.
From a viewpoint at the north end of the northernmost lake, the south face and west ridge will appear quite formidable with a steep, rocky south face and rugged looking west ridge. From the lake, head north, gaining about nearly 800 feet into a higher basin at the foot of the south face. Gain more elevation on a tundra-covered talus slope that leads through some cliff bands before it plays out. You'll have to doing some zigging and sagging through the rock bands, tending towards the west ridge. If you wish, you could work your way on over to the west ridge and then head east up the ridge to the summit. When we had opportunity to do so, we felt the south face route was going well enough to continue on it. About 300 feet below the summit, an eastward, upward sloping ramp leads over to the SE ridge of the peak. To get to that ramp, we did have to scramble (3rd class) through one narrow, rocky gully that had a darker colored rock. A later summer snow patch marked the ramp above. If you find and follow this ramp, you'll come out on the SE ridge, about 200 feet below the summit, at a small saddle with an astounding view of serrated, SE ridge of Castle Peak. Walk and still scramble at times on easier maroon rock to the summit.
This summit offers a fantastic view and different perspective of Castle Peak. After catching your breath, for bonus points, continue over to UN13,162 to the west by following the west ridge. It will not be an easy traverse from UN13,550 and may require anywhere from 1 to 2 hours to complete the 3rd class route with one section of ridge to descend on steep, exposed loose rock.