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Barbarian Invaders
From Discworld & Pratchett Wiki
A three-quarter-ton water-driven pub game devised by Leonard of Quirm. It was installed in the Mended Drum when the drawbacks with the Quizzing Device (the drinking clientèle kept getting arrested by the Watch, which was bad for business) became apparent.
Unfortunately its sole user so far has been the Librarian, who is developing worrying signs of what elsewhere would be called arcade machine addiction.
A pint of beer was poured into it at some point, and while it may be the imagination, one could almost swear that the invaders are wobbling more...
It parodies Roundworld's Space Invaders