supply depot vs watchtower

supply depot

noun
  • A facility utilized by armies to store battlefield supplies temporarily on or near the front lines until they can be distributed to army units. 

watchtower

noun
  • An observation tower in which a lookout keeps watch over prisoners, or looks out for fires, etc. 

How often have the words supply depot and watchtower occurred in a corpus of books? (source: Google Ngram Viewer )