Hiked into Castle Lake in the St. Helens monument area with my 9 year old son, a small raft, float tube and fishing gear, camping gear and food for 2 nights. It is a spectacular place - very remote, secluded, and scenic. We were alone for the whole weekend.
Check with the fly shop owner in Toutle for flies, map/directions and advice. This trip is not for everyone.
The hike is pretty dicey - there is no trail for the last mile or so and if you bushwack the shortcut route to a small inlet stream on the west side like we did, it is very steep. Do not do this in bad weather - there are cliffs and gullies with loose rock and lots of ways to get in trouble. There are lots of elk in this area which are fun to see, and in some cases have made some good game trails to follow. Naturally the hike out, uphill, is very difficult due to steepness, lots of deadfall timber, etc. Carry as little pack weight as possible. There is at least 2000 feet of vertical from the road to the lake, and coming out it feels like 4000 feet.
Having said that, you need a tube or raft to fish this lake well. The need for a float tube or raft, and camping gear, combined with the difficult trail, makes for a tough combination. Its too hard and far for a day trip, and you have to carry extra gear, making it that much harder. I recommend a small, one person lightweight raft (sevylor makes one) less than 8 pounds with a small paddle and pump. I would not bother with the float tube next time -too heavy, bulky etc.
Bank fishing is hard, sides are steep, lots of drifting logs. The fish are usually caught deep with sinking lines from away from shore. I caught some from shore on flies but shore fishing really limits your options.
We did not catch as many fish as I would have liked to make up for the effort required of the hike - probably 20 fish total. (not bad considering I was helping a 9 year old) Some were nice - 16" range. I believe the lake is over-populated. Reports of 5-10 pound fish are probably true but we never saw or felt one. But the overall experience and the beautiful setting made for a great, once-in-a-lifetime trip with my son that we will always remember.
If you go, do it for the overall experience of a unique setting and not expecting to catch 100 fish. And don't go alone. Scott