Castle Geyser is a cone geyser in the Upper Geyser Basin of Yellowstone National Park. It is noted for the particularly large geyserite sinter deposits, which form its cone. These deposits have been likened in appearance to a castle.
When the geyser was given this name in 1870, the top edges of the structure resembled the typical profile associated with the modern concept of a castle, having the appearance of a large keep, multiple turrets, and especially because of the crenellation along the top edges of what resembled its towers. As the drawing below shows the cone had distinctive appearance at the time. Over time the cone's shape changes because of the layers of mineral deposited in successive eruptions.
The Castle Geyser has a 10 to 12 hour eruption cycle. The geyser erupts hot water for about 20 minutes in a vertical column that reaches a height of 90 feet (27 m) before changing to a noisy steam phase that issues for 30 to 40 minutes.