Gloucester Cathedral, formally the Cathedral Church of St Peter and the Holy and Indivisible Trinity, in Gloucester, England, stands in the north of the city near the River Severn. It originated in 678 or 679 with the foundation of an abbey dedicated to Saint Peter (dissolved by Henry VIII). (source Wikipedia)
Gloucester's historic cathedral cloisters were transformed into the corridors of Hogwart's School of Witchcraft and Wizardry.
The Lavatorium on the left hand side of the North walk (yes, it's really called that and it wasn't the ancient Abbey monks' loo but the place they washed) is where Harry and Ron hide from a giant troll behind the pillars.
The troll in the film is mainly computer-generated, but huge legs were used in filming to make it look more realistic. They were so big that there was some difficulty in getting them in through the cathedral's massive main doors. (source http://www.bbc.co.uk/gloucestershire)
Daniel Radcliffe, Rupert Grint, Emma Watson
Harry Potter has lived under the stairs at his aunt and uncle's house his whole life. But on his 11th birthday, he learns he's a powerful wizard -- with a place waiting for him at the Hogwarts School of Witchcraft and Wizardry. As he learns to harness his newfound powers with the help of the school's kindly headmaster, Harry uncovers the truth about his parents' deaths -- and about the villain who's to blame.