The Castles is named for its coral formations, which rise in turrets and battlements above a network of gullies and trenches. Spirit of Freedom describes gullies and trenches running from about 30 m to a wall of more than 1,000 m, with groupers, triggerfish and gorgonian-filled overhangs. A ScubaBoard report from the Nimrod Explorer also lists 'The Castle' among Osprey's sites.

The topography makes it one of Osprey's more interesting sites to navigate. Gullies lead divers from the reef top down toward the drop-off, and the turrets between them are undercut by overhangs and small caves. Large gorgonian fans grow in the overhangs where the current brings food, and soft corals decorate the turrets. Beyond the edge, the wall falls away into the deep blue, so divers can finish the structural part of the dive with a look out into open water.

Groupers use the caves and overhangs as refuges, and triggerfish, including titan and clown triggerfish, are common. As at every Osprey site, grey reef and whitetip sharks patrol, and schools of trevally and barracuda may pass along the wall. Underwater Atlas groups The Castles with Silver City as signature sites with vertical terrain, abundant sharks and big-animal encounters, though detail in the published sources is limited.

The Castles suits intermediate divers comfortable with depth and some overhead cover in the gullies. It makes a good contrast to the plain walls at Osprey, with more to explore at moderate depths, and is well suited to wide-angle photography of fan-filled overhangs. Visibility is commonly 30 m or better.

Liveaboard only. Mike Ball's Spoilsport runs 4-night fly-dive trips (depart Cairns Thursday evening, cruise north overnight via the Ribbon Reefs, cross to Osprey, fly back from Lizard Island) and 5–7 night Coral Sea expeditions; Spirit of Freedom visits Osprey on its 4- and 7-night itineraries, with a Cairns–Lizard Island flight connection. The open-ocean crossing from the Ribbon Reefs to Osprey (about 100 km) is made overnight.

Allow 2–3 days at Osprey on a typical trip. The Castles is visited on Spirit of Freedom trips and occasionally by other Osprey liveaboards; it is dived from a mooring or tender drop.

Osprey is an isolated oceanic atoll about 350 km north-east of Cairns and roughly 100 km beyond the edge of the Great Barrier Reef, so every trip involves an overnight open-ocean crossing. Strong south-east trades (over about 25 knots, common June to August) make the crossing rough and can close the exposed eastern and southern walls; liveaboards then work the sheltered western wall and North Horn. Sep–Dec usually brings the lightest winds and visibility of 40 m or more.

Currents on the walls vary with tide and oceanic flow and can run strongly around the horns. Dec–Apr is cyclone season and trips can be cancelled or diverted to the Ribbon Reefs.

Descend to 25–30 m at the head of the gullies, look over the wall edge, then work back up through the trenches and between the turrets, checking the overhangs for gorgonians and groupers. Finish on the reef top for the safety stop.

Spirit of Freedom – 4- and 7-night trips including Osprey; also listed in a Nimrod Explorer trip report.