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4712 Troll on the Loose is a Harry Potter set released in 2002. It depicts the scene from the movie Harry Potter and the Philosopher's Stone where Harry Potter and his friends confront a Troll that has been let loose in Hogwarts Castle.
The set depicts a girls' bathroom in Hogwarts. The bathroom includes one sink with an exclusive faucet piece. The sink is hinged so it can fall over, simulating the Troll hitting it with his club. The room has one brown door with a key in it on the back wall and a working function that drops bricks on the Troll's head. The play feature is to have Harry use the function to drop the bricks on the Troll's head to knock him out so he can save Hermione (who is not included). The bathroom is designed to fit in between 4706 Forbidden Corridor and 4729 Dumbledore's Office, and with 4735 Slytherin on top.
The minifigures in this set are Harry Potter and the Mountain Troll. The troll is an extremely large creature, has a special bald minifigure head, and its lumpy body comes as a single piece, though one of its arms is articulated so that it can swing its club. The troll has unique cloth clothes and an elastic belt and flat horned feet. Harry comes in his 2001 Gryffindor Robes.