The Astronomy Tower is the tallest tower of Hogwarts castle. It is often simply called 'the tallest tower'.
Professor Sinistra teaches Astronomy on the tower. These classes always take place at night, when the sky is visible to be studied. The tower is out of bounds to students when they are not attending class.
In Harry Potter and the Philosopher's Stone, Harry and Hermione take Norbert the baby dragon to the top of the tower so Charlie can take it with him to Romania. This lands them in detention when Filch catches them.
|“|| The astronomy tower at Hogwarts is out of bounds to students except during classes, which take place at night so that students may easily study the movements of stars and planets.
Harry and Hermione bring Hagrid’s dragon, Nobert the Norwegian Ridgeback, to the top of this tower so that Charlie Weasley’s friends can collect the creature and take him safely away. After leaving the Invisibility Cloak on the tower, they are caught by Argus Filch.
From the Story
Discovered in Book 6, Chapter 27, The Lightning-Struck Tower
|“||The Dark Mark hovers over the Astronomy Tower at Hogwarts when Harry and Professor Dumbledore arrive back at the school. Draco Malfoy confronts Dumbledore on the Tower, eventually accompanied by four Death Eaters. Dumbledore is killed by Professor Snape, falling backwards over the Tower battlements after being hit by the Killing Curse.||”|