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This is probably the most popular walk in the whole park, and it certainly is spectacular. It's a bit of a slog though (the top of The Castle is more than 700m higher than Yadboro creek), and you'll definitely need a head for heights and some rock scrambling ability near the top. If you're freaked out by heights, this is not a walk for you as some of the bits near the top are quite exposed. On the other hand, you don't need to be able to rock-climb, just be comfortable hauling yourself up onto rock ledges.
Note: The map above is just a rough guide. The marked locations and route may be highly inaccurate. If you want to do this walk, get hold of the proper topographic maps and use them to plan your walk in conjunction with a reputable guide book.