Road and Trail ApproachThe trailhead for this climb starts at Trinity at the end of Chiwawa River Road.
To get to the trailhead, drive Stevens Pass Highway (US-2) west from Leavenworth for 14.5 miles or east from Stevens Pass for 19 miles. Turn north off of the highway at Coles Corner (a gas station and restaurants are at this corner). Drive Lake Wenatchee Road northward for 4.4 miles to a wye just beyond a bridge over the Wenatchee River. Go right at the wye and drive 1.2 more miles to Chiwawa River Road on the left. Take a left. Continue up Chiwawa River Road for 23.5 miles (paved for first half) to its end at Trinity (c. 2,780 ft). Phelps Creek Campground is on the left before the trailhead.
Hike the Buck Creek Trail for 2 miles to a junction (3,200 ft) and take a right onto the Chiwawa River Trail. Hike the Chiwawa River Trail for 3.5 miles to another junction (4,760 ft). The left fork descends to the head of the Chiwawa River. The right fork ascends the west side of Red Mountain in a series of switchbacks then contours north to an end in about 2 miles (c. 6,400 ft). One can take the left fork or right fork to carry on to Fortress Mountain. The left fork is more direct but loses elevation and may encounter brush. The right fork is longer but stays in lovely sub-alpine bench land.
Off-Trail ApproachAs stated in the previous paragraph, there are two off-trail approaches to the east side of Fortress from trails' end.
Option 1: From trail to head of Chiwawa River
From the trail junction at 4,760 ft previously mentioned, take the left fork. This trail goes about 0.7 miles to a long leftward switchback at 4,820 ft. Leave the trail here and cross the creek bearing northwestward onto the minor timbered ridge that forms on the other side. Ascend this ridge for less than a mile to a meadow at 5,500 ft immediately east of Pt. 5971. From this meadow, simply take the path of least resistance up and left to the alpine environment above. Once you get to 6,400-6,600 ft you should be able to see the rest of the route to the base of the East Face of Fortress. Simply bear toward it (northwestward). There will be a number of drainage gullies for the upper snowfields to contend with but you should be able to manage to get through or around them. From where you leave the trail, give yourself 3 hours to reach the base fo the face.
Option 2: From trail on east slopes of Red Mountain
From the trail junction at 4,760 ft previously mentioned, take the right fork. This trail goes ~2 miles to an end at about 6,400 ft west of Red Mountain. The end of the trail is defined by a steep gravelly/rocky "road" to an old mine. There may be a bootpath beyond here that ascends to the 7,300-ft saddle north of Red Mountain. That would be a cross-country route to Spider Gap. Leave the trail just before or on the gravelly road and descend a couple hundred feet and cross a basin to gain the benches on the southeast side of Chiwawa Mountain. Travel westward along these benches (adjusting your elevation upward as you go until it is possible to round the corner of the base (c. 7,000 ft) of the prominent South Ridge of Chiwawa. At this point the East Face of Fortress should come into view again. Simply bear toward it (westward) keeping as high as possible to avoid unseen gullies farther down. From where you leave the trail, give yourself 2+ hours to reach the base of the face.
The East Ridge RouteEither off-trail approach option should bring you to a sizable snowfield below the East Face. For the second option above, to avoid the gullies lower, you will probably find yourself crossing a permanent snowfield. This snowfield steepens at its west end where it makes a descent into the snowfield under the East Face. Ice axes mandatory. You can stay high to avoid the steep part, if necessary. The snowfield below the face turns to talus in late season in its upper reaches.
From the snowfield or its east fringe, climb northward to reach the East Ridge just west (8,400 ft?) of a bulky crag. Follow the crest westward on bouldery Class 3 terrain until you reach a short step. A 10 or 15-ft Class 4 chimney gets one above the step. It's not that exposed except for the 15 feet from its top to its bottom and you should therefore be able to free climb it up and down. Once above the step, simply climb up and left on more Class 3 terrain until the summit "plateau" is reached. The highest rocks are a short distance to the southwest once you get onto the plateau.
You can descend the way you came up.
Caution: Stay away from the East Face at all cost. I made the mistake of climbing up it (I came down the East Ridge) and almost killed myself. The face is comprised of loose rocks and steep sandy slabs and benches.
Essential GearIce axe; possibly crampons if later in the year. I did the climb in September without crampons but remember thinking they would have been nice to have.
Bicycle if you want to ride the first few miles of trail. You're not supposed to ride the trail but it would go three times faster.