In this tower the Astronomy classes are being held. But, believe it or not, that is not their most important function.
This tower is also one of the most romantic places in whole Hogwarts. At night, you can see the moon and the stars through the half-open roof. During the day, it is quiet and peaceful here, and you have an awe-inspiring view over the whole terrain.

Where Chase does his work when he doesn't have a class to teach, his bedroom is attached.

The Owlery is a circular stone room, rather cold and drafty, because none of the windows have glass in them. The floor is entirely covered in straw, owl droppings, and the regurgitated skeletons of mice and voles. Hundreds upon hundreds of owls of every breed imaginable are nestled here on perches that rose right up to the top of the tower, nearly all of them asleep. Shh! don't wake them!