The Astronomy Tower is the highest tower at Hogwarts. In Harry Potter's first year, he and Hermione Granger climbed to the top of the Astronomy Tower to give Rubeus Hagrid's pet dragon, Norbert, to Charlie Weasley to take care of. Five years later, when the Death Eaters arrived at Hogwarts, most of the Battle of Hogwarts between the opposing sides took place in this tower, or in the corridor below.
There was a battle at the top, involving Death Eaters. Albus Dumbledore was blasted from the tower's ramparts by an Avada Kedavra from Severus Snape. This is where he met his death. This was predicted by Professor Trelawney when she was feverishly reading her Tarot cards. In 1996, she mutters, "The Lightning-Struck Tower - an omen ..." She was unwittingly referring to the Astronomy Tower, in which Dumbledore met his death.