London, 21/04/2015. Today, the King's Troop Royal Horse Artillery have fired 41-gun royal salute to celebrate the 89th birthday of Her Majesty the Queen Elizabeth II at Hyde Park. << […] The basic royal salute is 21 rounds. A further 20 are fired at royal parks and royal palaces and fortresses and another 21 for being in the City of London. […]>>, (Source - BBC.co.uk).