Note: This route description assumes the use of a 4WD with good clearance to reach the trailhead. If you only have use of a standard passenger vehicle, add 1.4 miles one way to the distance and another 350 feet of elevation gain.
From the trailhead parking, head south of Forest trail #226, which eventually goes over a pass into Wetterhorn Basin, but you will not be going that far. There is a trail register at the start. The trail leads through mostly open meadows the first half mile rife with wildflowers at the right time of the summer. As you're walking along, be sure and identify which peak is Fortress and note its west ridge that drops down to the valley floor. You want to make sure you walk south up the trail far enough to be past that west ridge. That will mean a distance of about 1.2 miles. As the trail begins to enter and stay in some trees, you'll cross one drainage which may or may not be running and then another. Cross both, but a little past the second is where you'll need to leave the trail and head generally east on the south side of that drainage. This is the first drainage south of the west ridge of Fortress. If you have GPS, these coordinates should help identify the approximate spot: N 38° 05' 52.72" W 107° 32' 55.48". You don't have to be in the exact spot.
Bushwhack east along the side of the drainage/creek for a while. You may find a cairn or two and faint trail. After a few hundred yards, it may become easiest to enter the drainage and walk in it if dry or running low. Continue up the drainage until you see a dry fork coming in from the left with rocks, tundra and grass. Head up that way. If you're in the right place, above the head of this dry fork, you'll be looking directly at the large, cliff/buttress on the west ridge of Fortress. Do not try to continue in the main drainage. In this left fork, there will be some rocks & boulders to get around and it is fairly steep. As you hike upward, you'll notice a ridge on the right side with some trees. Exit this drainage just above where the trees end to gain the ridge. Walk steeply uphill now on grassy tundra toward the lower cliff band. There will be two possible breaks in the cliff from which you can choose. One almost directly above and the other a little further to the right. Either one will get you through to a sloping bench area below the second cliff band with no more that a brief 3rd class scrambling move.
Once below the second cliff band, in the immediate area, look for another break close by that allows a brief 3rd class scramble again for about 30 feet and takes you through the cliff. There are several places in the cliff that will seem to offer a possibility, but this spot will appear as the easiest. It is fairly narrow. Exposure is minimal. You may want to send people through one at a time. Once above, the terrain changes to more sparse vegetation and small scree & talus. Note that it is also possible to walk along the sloping bench below the second cliff band eastward until the cliff plays out. You can then just walk on up and head back over to the west ridge.
Once through the two cliffs, it's steep but easy walking to the summit on mostly small scree. Some of the drops to the north off the ridge offer nice views. We've observed a pair of golden eagles flying here. Ahead, looking up the ridge, you'll see the summit block guarded with a sheer cliff. Don't worry - it's easily circumvented. As you approach the summit, one brief section of the ridge will narrow and drop off on either side. Beyond that, it's a stroll to the summit.
On the summit, with a well-built summit cairn, if you look north, there's another summit just a short distance away that appears as high. No one to our knowledge is claiming that it is, which is probably good. To get to that point, you would have to come back down from the main summit a little, walk gingerly across and sharp, rotten ridge and then scramble up a very rotten looking conglomerate 15 foot wall. No thanks! If you're interested in either Precipice or Redcliff, you'll have good views either way and of the connecting, gnarly ridges. A report from someone who has done or attempted to do either of those connecting ridges would be interesting. We've heard rumors but nothing more.
Links to other information, routes & trip reports for this peak that may be helpful.