Answer: periscopes

AnswerCrossword Clue
PERISCOPESKind of peepholes
PERISCOPESU-boat features
periscopesAn apparatus consisting of a tube attached to a set of mirrors or prisms, by which an observer (typically in a submerged submarine or behind a high obstacle) can see things that are otherwise out of sight
periscopesPERISCOPE, a device (eg a tube with mirrors) by which an observer can look at something which is not in the direct line of sight