Why were big holes made in the bastions?

Bastion, also known as Burj, was present in large numbers in the Golconda fort. The walls of the fort were made to project outwards in a circular fashion to make a burj. Now, these bastions also had holes in their walls. In times of war, their massive height almost gave an aerial view of vast lands, thus warning them of any possible invasion. And the holes allowed them to attack the invaders without being seen.