I have hiked Castle a number of times, the first being back in the early 80s when I really wasn't into hiking at all. On that occasion, I joined two friends, Bob Benham and Guy Kaulukukui, who both worked for KSBE at the time and hence were able to get access.
During that hike of Castle, I remember a particularly dicey section where we had to inch across an exposed waterfall section of the old switchbacks. At the time, I thought that was the craziest thing I had ever done in my life.
I also remember only carrying a liter of water and drinking water out of the stream that the trail crosses what up in the mountains above Punaluu. At the time, I reasoned that the water had to be pure since no animals who could foul the water could exist this high up in the Koolaus. Of course, years later I found out how incorrect I was and that indeed there were lots of animals who live up that high (and higher) and they had no problem fouling the water.
Fortunately, I survived that drink-from-the-stream episode and lived to hike another day, which included other days in the years that followed hiking on the Castle Trail which is mentioned favorably in Stuart Ball's The Hikers Guide to Oahu.