Ruined Castle, a rocky outcrop perched on a rise in the Jamison Valley, can actually be seen from the famous Echo Point at Katoomba, but you may have missed it alongside the more famous sisters.
You can either start the walk at Scenic World - probably the best option if arriving via public transport, or from the "Golden Staircase" near Narrow Neck which is best reached by car. The walk is well-maintained and sign-posted so it's manageable even without a lot of hiking experience, but allow the best part of a day to make the return trip.
From Scenic World you can take the Scenic Railway up and down which cuts out the most demanding part of the trip - the steep uphill climb up the cliffs, while the "Golden Staircase" rewards your extra efforts with stunning views of the Jamison Valley.
The walk from the Golden Staircase is almost 4km each way, and takes you for the most part through cool forest where you can find a wide variety of wildlife, and partly through the rocky outcrop where you'll be rewarded with panoramic views from this unique natural formation. Bring plenty of water and a picnic, and take the time to enjoy this tranquil area of the National Park.
Take a look at the official page of the park for latest updates and closures, and check out my other articles for more outdoor ideas.