On November 16, 1961 Netherlands Antilles issued this 20c stamp (Sc #273) to commemorate the 185th anniversary of the first salute by a foreign power to the USA. There are two cannons shown at the fort, and a larger cannon on the cancellation.
From wikipedia, "Sint Eustasius was the most important place for dealing or smuggling with Americans. On 16 November 1776, at the beginning of the American Revolutionary War, the American navy ship Andrew Doria
, with the American Declaration of Independence on board, arrived at St. Eustasius. Its captain fired a salute to the Dutch flag on Fort Orange and Johannes de Graaff decided to answer the salute with eleven gunshots. And so the United States of America were for the first time recognized as a nation by this salute of eleven guns."