Tower Of London
Though most famous for the imprisonments and beheadings held there it has, at various times, served as the royal residence, mint, menagerie, armory, observatory and as the holding place for the Crown Jewels - a function it still serves.
Legend has it that six ravens must guard the tower at all times, or else the kingdom (as in United Kingdom) will fall. The ravens are still there, and so is this nearly thousand year old complex of stocky stone structures.
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So much to experience
Alow plenty of time