Mt. Carru is an important mountain closely west of higher Mt. Lago. Closely west of Carru is 8,587-ft Osceola Peak. The Osceola-Carru-Lago line of peaks cannot really be said to lie along a 'ridge' as the clefts between the peaks are quite pronounced. The saddle between Osceola and Carru is approximately 1,100 ft below their summits while the prominent notch between Carru and Lago is approximately 1,000 ft below Carru's summit. Because of these prominences, Carru is quite a solitary mountain bookended by two other Washington Top 100 peaks.
As a climb, the mountain is not that interesting. It's just a scree and talus heap with many parallel ribs. Ascent time base to summit would be less than two hours. As a viewpoint, however, it is outstanding.
This area of Washington is rugged but not as much as more glacially active regions to the west and southwest. Because of this, approaches to the Mt. Carru area can be made from numerous directions. The shortest of these from a road would have to be from Slate Pass (on Harts Pass Road) down the Middle Fork Pasayten River then up to Freds Pass and across the upper reaches of Eureka Creek, but even this approach is 15 miles long. Another approach would be via Monument Creek Trail (again from Harts Pass Road). This route is 19 miles or so in length and is more up and down than the Slate Pass approach. Other approaches are from the north, but only Canadians would come from that way.
Permits are not required as far as I know, though signing a trailhead register might be requested. You won't run into much civilization out in the Pasayten Wilderness. When I was out there for a week in August, I saw less than five people once I got back to the Carru vicinity. You'll see more people (and horses and dogs) on the Robinson Creek Trail, which runs along the Middle Fork Pasayten River north of Slate Pass.
When To Climb
Mt. Carru could probably be climbed in every season except winter. Access depends on conditions for getting to the trailhead. The Slate Pass trailhead is ~6,200 ft up. In fact, the road to Slate Peak Lookout is the highest well-traveled road in Washington. Harts Pass Road is also one of the more exposed roads to drive on. Combine that with the many sightseers who use the road and it becomes a concentrating endeavor. The snowier the approach, the longer time you'll need to get in and out.
If you're going to go climb Mt. Carru, you could probably get in and out in two days, but three days would be more leisurely. And actually, you might as well make it a multi-day trip and climb other peaks in the area. Each peak (and the region in general) offers excellent vistas--especially for photography.
There are numerous campsites. Some I know of are:
1. At the intersection of the Robinson Creek Trail and Berk Creek (this is where the trail up to Freds Lake, Freds Pass, and Lake Doris begins)
2. At Freds Lake
3. At Lake Doris
4. At the eastern head of Eureka Creek immediately south of Mt. Lago
5. At a half-acre sized bench at ~7,500 ft just southwest of Shellrock Pass. This is where I camped for five days. It is a good staging area for climbing all the peaks in the region. There may or may not be a snowpatch there for water. Bring a filter.
When I was in there in August 2001, I was snowed on lightly for one night at my 7,500-ft camp. Knowing this, plan your clothing appropriately. It can be hot or cold or in between in the Pasayten. Ordinarily, you won't get much rainfall as the wilderness is far east of the Cascade Crest. However, when I was there, it rained for 36 hours straight. I stayed in my tent nearly the whole time.