There's something dry about the Tower of London (4)

I believe the answer is:

moat

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But I can't explain it at all! Could you help me understand?

(Other definitions for moat that I've seen before include "Water around castle" , "A deep water-filled ditch around a castle" , "Water-filled trench around a castle" , "Water-filled defensive trench" , "Water crossed via a drawbridge" .)

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