The Viaduct was a bridge at Hogwarts Castle. It was constructed of stone, spanned a large chasm, and connected the grounds with the Viaduct Courtyard. There is a safety balustrade made of stone on each side of the Viaduct walkway to prevent people from falling, as well as iron torches to provide lighting.
The viaduct is one of the few areas of Hogwarts which has stayed consistently the same throughout the first six film adaptations. However, in Harry Potter and the Deathly Hallows: Part 2, the Viaduct changes places, this time connecting a bigger Viaduct Courtyard to some nearby hills, part of the Forbidden Forest. There also seems to be a second viaduct that is still connected from the corner of the courtyard to the entrance between the Left and Right Viaduct Towers. This likely happened as the development of the Battle of Hogwarts requires a direct connection from the grounds to the main entrance and enough area for many occupants to battle. However the Viaduct possibly always there in the start of 1994-1995 school year, as the Hogwarts carriages can go to to the Entrance Courtyard during the Yule Ball in the Harry Potter and the Goblet of Fire (film) and during the term, Viaduct are concealed by magic for the safety of students.
In the films, althrough it appears in all films, except Deathly Hallows: Part 1, the only time it is seen used is in Deathly Hallows: Part 2 during the Battle of Hogwarts. The same goes for the Boathouse staircase.
