In the 19th century, cannon were mounted on the rooftop and fired on ceremonial occasions.
One was used regularly as the ‘noon gun’, firing at the midday in response to a telegraph signal from Greenwich Observatory in London. After a number of accident – in one of which a gunner was killed – the practice of firing the cannon was eventually discontinued in the 1850s and the guns were later removed.*
*Information from Newcastle Castle guidebook (Society of Antiquaries of Newcastle upon Tyne).