The Castle Group encompasses only a few features around Castle Geyser. On its west side the old road between Old Faithful Area and Morning Glory Pool intersects the group, to the east Firehole River marks the border with Sawmill Group.
Castle Geyser is exhibiting one of the most massive geyserite cones worldwide. It is 3.7 m (12 feet) tall, 36.5 m (120 feet) in circumference at the base, and 6 m (20 feet) in diameter on top. The 90 cm (three feet) wide orifice has the same diameter as the one of Old Faithful Geyser. Radiometric dating of the cone revealed an age of approximately 1000 years, far younger than assumed before, while the surrounding sinter shield turned out to be around 10,000 years old. The early explorers of Yellowstone were not only impressed by Castle's size, but also by the exquisite globular ornamentation all around the cone. Until 1880 visitors chipped off most of the geyserite nodules, leaving behind the plain surfaces we see today.
In contrast to White Dome Geyser or Lone Star Geyser the size of the cone actually goes along with the power of Castle's eruptions. Water jets may reach up to 30 m (100 feet) height. However, Castle's play and intervals are known to vary within broad limits. Most common are major eruptions, followed by a longer steam phase, at an interval of approximately 11 hours.
At the base of Castle's huge sinter cone sits Tortoise Shell Spring. It is constantly boiling and splashing a few feet high.
Gizmo Geyser is another feature at the base of Castle Geyser. It plays from three crack-like vents and a small open pool furthest away from Castle. Intervals seem to be quite variable and it is also known to act as a perpetual spouter or to undergo longer periods of dormancy. The geyser's name sounds rather weird. It is said that around 1990, when geyser activity was new to this feature, somebody in a group of observers called it "little Gizmo", much to amusement of all, and the name stuck.
The small, unnamed spouter next to Gizmo shows significantly stronger play if Gizmo is not dormant.
Across the road from Castle Geyser the drop-shaped Shield Spring appears.
Beyond Castle Geyser a boardwalk branches off downslope to Firehole River and passes Crested Pool, a typical bubble-shower spring. A tragic accident happened at Crested Pool in June 1970, before the turn of the boardwalk in front of it has been secured by a handrail. Blinded by steam, a 9-year-old boy from New York stumbled into the spring, died in the 93 °C (200 °F) hot water, and sank out of sight before the eyes of his parents. His disintegrated body could not be recovered completely, so Crested Pool actually became his grave.
Next to the eastern corner of Crested Pool's observation platform Tilt's Baby shows up. Tilt Geyser, located right of it, is dormant since long time and its crater is all but invisible. In contrast, Tilt's Baby erupts every 4 - 6 hours up to 2 m (6 feet) high. Standing on the platform, it is easily possible to look vertically from above into the erupting geyser, but don't do it! You never know, how high the eruption will be (Update: The observation platform has been relocated several feet away from the geyser in 2016, so the risk is eliminated).
Near the end of the boardwalk from Crested Pool down to Firehole River little Dungeon Geyser (UNNG-CGG-10) lies almost hidden behind some small trees in downstream (western) direction. It is rather inconspicuous; so to find it for the very first time look for the feature across the river from South Scalloped Spring. The dungeon of Dungeon Geyser undermines a small sinter mound and exits at several openings. One of them faces east, two lie on the west side of the mound, another one towards Firehole River. Even if it would make sense, the name doesn't refer to the branched plumbing system, but to its position on "dungeon level" of Castle Geyser. Spotting one of the frequent eruptions needs sharp eyes since they are only inches high and the geyser is located several feet away from the boardwalk.
Downslope, on the bank of Firehole River, lies a nice, but quiet and unnamed hot spring cone.
The bridge over the river provides a good look at the Terra Cotta Geysers a little bit farther downstream. They are encompassing at least six different geysers, named consecutively from Terra Cotta "A" to "F". Here, we took over the letter designations and names as set by geyser expert Mike Keller (GOSA Transactions XI, 2010). Most often of all, as to be seen on the first picture, Terra Cotta Geyser A is active at intervals of approximately 2 hours. It is known to erupt sometimes together with Terra Cotta E, to be seen on the far lefthand side on the pictures. Just in front of "E" the small cone with two openings of Terra Cotta F is located. Like "E" it may join in eruptions of "A" or may play independently. Keller identified 7 further vents (designated as Terra Cotta "G" to "M") of Terra Cotta Geyser A, which are encircling the main vent within the margins of its crater and may erupt together with "A".
The next photo shows an eruption of Terra Cotta Geyser B, also called Dishpan Spring. At an interval of 3.5 to 8 hours its play is less frequent than "A". The pool in center is Washtub Spring (Terra Cotta C, often dormant), and the pool far left is Terra Cotta D, the rarest playing geyser of all.
Also on the south river bank, but 150 m northwest beyond the Terra Cotta Geysers, the perpetual eruptions of Spanker Geyser can be spotted.
Just in opposite direction (upstream) from the bridge Sprinkler Geyser is to be found next to the river downslope of Castle Geyser. It is best observed from a spot near Liberty Pool on the trail between Sawmill Geyser and Lion Geyser. Sprinkler is erupting very frequently and up to 3 m (10 feet) high.