Beleriphon
Totally Awesome Pirate Brain
There are enough incidents in history where there was a "BIG BANG" as a result of stored gun powder, and enough still extent ruins where the damage from the explosion is observable in all or in part, that I think we can establish that for enough weight of dry, well stored, finely ground and well compounded gun powder, there is in indeed a "BIG BANG". It won't look like a Hollywood explosion (which is typically mostly gasoline), and it won't be as brisant as an explosion from a high explosive, but it will still lift the walls of your star fort into the air or turn your giant man-of-war into a pile of splintered debris.
That only happens if the explosion takes place in the powder magazine, which was usually deep inside a fort to prevent accidentally lighting it off during a battle. If it was though, it would blow a hole in the walls. However, just setting a keg of powder off on top of a wall wouldn't actually do that much damage because all of the pressure just goes up into the air. Even modern high explosives wouldn't do much if you just strapped hunk of C4 to the side of a star fort set it off.
Heck, even a middling sized nuclear weapon wouldn't actually do that much to a start fort, unless is was dropped right on top of one, and that's largely the heat rather than the pressure wave.