Use 'in ruins' to emphasize the visible devastation of something, often with emotional effect. The ancient monument now lies in ruins, a ghost of its former glory.
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Historical Context
Apply 'in ruins' when discussing historical buildings or cities that have been destroyed in the past. The castle stood for centuries but is now in ruins.
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Metaphorically Speaking
'In ruins' can be used metaphorically to describe something that is not physically destroyed, like plans or hopes. After the failed merger, their expansion plans were in ruins.