" No enemies had ever taken Ankh-Morpork. Well technically they had, quite often; the city welcomed free-spending barbarian invaders, but somehow the puzzled raiders found, after a few days, that they didn't own their horses any more, and within a couple of months they were just another minority group with its own graffiti and food shops. "

― Terry Pratchett , Eric (Discworld, #9; Rincewind #4)