The astronomy tower is the tallest tower of Hogwarts. This is where students had Astronomy class every Wednesday night at midnight during Harry’s first year
The top of this tower where observation sessions are held for Astronomy is more or less directly above the front entrance of the castle; the top of the tower has a parapet (OP31).
A steep spiral staircase leads to the top of the tower (PS14, OP31), with a door (which has an iron ring as a handle) leading out onto the ramparts, which are crenellated (HBP27).
To get from the foot of the Astronomy Tower’s staircase to the staircase leading to the entrance hall, one must pass through two corridors before rounding the corner to see the staircase leading (eventually) to the Entrance Hall (HBP28).
The Tower is handy for meeting with broomstick riders in the dead of night - except when it requires hauling a heavy crate loaded with dragon up the three flights of stairs from the Entrance Hall to the corridor beneath the tower (PS14).
The Battle of the Tower was fought at the base and on top of the Astronomy Tower. Dumbledore's lifeless body fell from there to the grounds below (HBP27).