From Glossary of Meteorology
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(Previously called castellatus.) A cloud species of which at least a fraction of its upper part presents some vertically developed cumuliform protuberances (some of which are taller than they are wide) that give the cloud a crenellated or turreted appearance.

This castellanus character is especially evident when the cloud is seen from the side. The cumuliform cloud elements generally have a common base and usually seem to be arranged in lines. The species is found only in the genera cirrus, cirrocumulus, altocumulus, and stratocumulus. Cirrus castellanus differs from cirrocumulus castellanus in that its vertical protuberances subtend an angle of more than 1° when observed at an angle of more than 30° above the horizon. When altocumulus castellanus and stratocumulus castellanus attain a considerable vertical development, they become cumulus congestus and often develop into cumulonimbus. Stratocumulus castellanus should not be confused with stratocumulus pierced by cumulus.
See cloud classification.