If coming from either the Front Range or the Western Slope, drive to Gunnison. From Gunnison, drive north on SH135 to Crested Butte. Stay on this main road as it enters the town, driving on through a major intersection with stop signs. Continue north on what becomes CR317 to the base of the ski area which is the Town of Mt. Crested Butte. Continue on the same road and a little after the main village and passing numerous condos, etc., the road will change over to a graded dirt road. During summer months, water trucks douse the road to help hold down the dust. Driving this road just after one of those trucks has gone through can change the road to a squishy, muddy mess, briefly. Continue on CR317 to the site of Gothic which has a small but nice visitor center where you can inquire about parking regulations, hiking and biking trails and the research that is conducted in this area. During summer months daylight hours, this road is heavily travelled and parking is restricted to designated areas.
Continue driving NW past Gothic another 2.5 miles to where the road for Rustler Gulch heads off to the right (north). This is FR317.2B on CalTopo. The Gunnison National Forest map does not really show this road. This road is less than a half mile past the "Gothic Campground." If in a passenger car, park in the vicinity of the turnoff. There's no sense in trying to get any further without 4WD. If you do have 4WD and the East River is not in high stage runoff, drive on down the rocky stretch through some close, tall willows and ford the river. Once across, the road will head steeply uphill for .7 mile to an intersection where you'll see a gate across the road that heads off to the right. Park here if there's any room to do so. Two or three vehicles might be able to fit in off the road just enough to let other vehicles by. Coordinates for this spot are: N38° 59' 57.31" W107° 00' 11.05". There's a stand of tall willows here.
Right after where FR317.2B crosses the East River, there's an area that can accommodate a limited number of vehicles and some primitive camping right off the road. Then further up the road a short distance is another primitive spot that could accommodate a single vehicle on the right.
For camping further away, try the Gothic Campground about a half mile before FR317.2B or some other primitive spots either back toward Gothic or further up the main valley of the East River.
For this "approach" there are two possible start points. If in a passenger car, the start will be where FR317.2B turns off of FR317 to cross the East River. You will then need to walk up FR317.2B to the trailhead coordinates. If you have 4WD, the start will be about a mile up after having forded the East River at the trailhead coordinates provided. There's a fence line and gate across the road/trail there.
From the gate, walk north on the old road bed. In July, you'll enjoy a plethora of wildflowers that will distract you from the hiking. Heading up the well-used trail, walk through an open meadow before entering a forested section with a considerable amount of bog, exacerbated by the horses that pass through regularly. Gingerly walk through, trying not to get too muddy this early into the backpack and continued on into open, flower-filled meadows that prompted us to frequently stop and take photos. This was just a taste of what awaits in upper Rustler. In short order, you'll come to a creek crossing that probably will require boot removal. However, a trail heading off to the right along the creek into the woods and willows goes to a place where you might be able to gingerly cross on some small logs and get across the creek. Once across, this side trail continues and eventually leads back to the main trail, but not before taking you through an incredible meadow of more wildflowers where you may want to spend even more time taking photos.
From this meadow, the open trail continues north and then begins to turn east into the upper basin. Continue up to where the trail has decidedly turned east and stop at a place where the creek, not too far below, cascades over some rock outcrops. Walk down and hop across the creek and hike up into a stand of trees and explore around. There was a faint game trail through here and a lot of fallen timber. Head upward and eastward through the timber. Near to where you exit the timber on the east side is a campsite, just the right size for two tents, with an old fallen log for cooking and eating and an old fire ring. See our approximate coordinates. This makes an excellent base camp for the six 13er summits you can access from this valley and is out of the way of the day hiking foot traffic that frequents this valley. A game trail of sorts leads east from the campsite, then turns north through lower willows back down to the creek for an easy crossing and to regain the main trail.
The upper basin of Rustler Gulch is so verdant and filled with wildflowers, you'll not likely find any campsite unless you want to trod down a bunch of flowers. The camp location described above is the only really decent campsite we found that would do minimal damage to the abundant wildflowers.
We climbed Cassi Peak as part of a 3-summit day that began with Precarious Peak. To reach Cassi from Precarious, we descended the SW couloir of Precarious until we could exit it and then begin a trek along the bottom of the cliffs on the south face of Precarious as we headed generally SE and then south to access Cassi. It is not required to climb Precarious in order to reach Cassi, so Cassi could be done alone or in combination with UN13,260 from Rustler Gulch. However, the mileage and elevation gain estimates provided assume a climb of Precarious first and then descending the Precarious SW couloir and traversing over to Cassi.
For Cassi Peak, we swung to the south some from the base of the cliffs on the Precarious south face and then began ascending along a route that took us by three snowfields where we could take a break from the rocks. The soft snow was not much easier to gain elevation on though, so we resigned ourselves to the monotonous struggle up the rubble and loose boulders that led all the way to the summit. Our route basically had us gaining the north ridge of Cassi about 200 feet below the summit and following along the ridge to finish. It was not a pleasant peak or ascent. The rubble never relented. There's any number of ways you could obtain this summit on the west side, but all of them will be rocky unless you come early season and find more snow than we did. In that case, an ice axe might be useful.
To descend Cassi to the saddle between it and UN13,260 we followed the SW facing, connecting ridgeline until forced off of it and into two distinct gullies on the west side of the ridge. The first gully was a short one, but very steep and loose and you must take care in descending it. This leads to a broader and easier gully that will dump you out a little below the saddle that connects the two summits. Negotiate this final gully without too much problem, and then regain the saddle. From the saddle, either head west back down valley or continue on to UN13,260.