The throne room is visited by thousands of people every year, but few are likely to know that below it lies a more humble gallery that would once have housed guards on constant lookout. More mysterious are the networks of tunnels and caverns beneath the Alhambra complex. Some may have been used to store grain, others were clearly dungeons. Historians think they may have been used to house important prisoners for ransom or exchange.

Now check out the mysterious stone circles still baffling experts

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21 iconic buildings that are hiding fascinating secrets