There are two ways to go up that we did: one is by way of the Papali Trail, and one is by way of Waiahilahila on the side of the Sacred Falls valley.
Take Papali Trail for about an hour. The trail comes to a ridge and it goes to the left of the trail. At that point leave the main Papali trail and go straight up the ridge. This ridge was cleared wide back in 1989, but by now is no longer wide. After maybe 2 hours it arrives at a junction with the trail from Waiahilahia.
The alternative is to take Sacred Falls Trail, and branch off to the right. After 15 minutes or so, look for ribbons on the right, at which point we go up the ridge to the steep hillside of Waiahilahila, which we simplify to "Nipple", because of its shape. It takes a bit more than an hour to climb to the top of Nipple, and it is very steep, and there are no cables to help.
From the Nipple, go down on the other side slightly and then climb again. After an hour and half, there is a steep climb, at the top of which is the junction with trail coming up from Papali.
From either spur, continue on the trail towards the Koolau summit. The trail is always on the ridge top. After almost three hours, we arrive at a high point, where you can see part of Castle Trail etched into the hillside. Then the trail goes down somewhat, and you reach the junction with Castle Trail. It takes something close to 6 hours to get here from either trailhead.
Take a left turn on Castle Trail, which switchbacks downhill to the stream. We reach the stream in an hour. Cross the stream and follow the definite Castle Trail and in a half hour we come to the cliff face on which Castle made the spectacular switchbacks going up from Punaluu Valley. Follow Castle down the cliff-face. Unfortunately many of the switchbacks were overgrown, and the searchers hacked through a ridge trail short-cut going straight down the cliff-face, rather than clear the overgrown switchbacks. Follow that short-cut, and cross Castle 4 times. Finally the short-cut stops and we rejoin the Castle switchbacks way down.
Simply follow Castle trails all the way down. It is maybe two hours from the top of the cliff-face. At the bottom there are some fierce dogs, and you may want to grab a large branch for possible defense.