Fortress is one of the tallest mountains in the state and offers incredible views of the surrounding Entiat range as well as majestic Glacier Peak and the North Cascades.
Starting at the Trinity trailhead you'll hike the Buck Creek Trail towards Buck Creek Pass. The trail is fairly obvious once the snows have melted but if you are going early season (June-early July) you will definitely want to bring a GPS. The trail works it's way up the valley along Buck Creek. You will pass through a burn zone from the Buck Creek fire (2016) and a impressively large Avalanche path. It is almost 9 miles to the turn off for Fortress.
Note: please only camp in established spots as the alpine environment is fragile. Also please respect all regulations on fires. Just because there is a fire ring doesn't mean you are allowed to have a fire.
About half a mile before Buck Creek Pass you will come to a creek. There is an obvious boot path up and to the right before the creek. This is where you leave the maintained trail and the adventure begins! Follow this trail up into a basin above. There are several good spots to camp in the basin below Pass No Pass which can be seen once the trees open up. Continue towards Pass No Pass. You don't need to make it all the way to the pass as you will cut back to the right up the steep grassy slope. You will stay on the south side of the west ridge of Fortress and continue ascending until you reach a flat bench. This is where one could bivy for the night if inclined.
Continue up and head towards a prominent spire along the south ridge of Fortress. You will traverse to the left under the spire and start ascending on loose terrain. It's more or less choose your own adventure from here as you make your way higher and higher. It shouldn't be harder than class 2 3 if you're on the easiest route. Be careful not to knock the loose rock down on your party or any others. Helmets are advised for this section. Continue up and to the left. Don't gain the summit ridge too early or you might have to down-climb a bit. The looseness tapers off near the summit.
You've made it! After enjoying the view head back down the way you came making sure not to inflict rockfall to anyone below.
There is a 4th class route down the east side of Fortress that would allow you to go tag Chiwawa on the same trip. This route is more difficult and only advised for experience scramblers and mountaineers.
This is a great side trip for Backpack Spider Gap to Buck Creek Pass. It can be done on the last day as you head out to the Trinity Trailhead. This is a long but rewarding day.