I was reading that the most effective way of attacking a star fort was to build offensive trenches parallel to the fort and then have zigzagging ones approaching it in order to stay out of fire from the fort and get close enough to bring the attackers cannons into action. But it seems like the trenches would still be vulnerable to mortar fire, were star forts loaded with tons of mortars and a huge amount of ammo that tried to knock out enemy cannons, or was this not as effective as I'm thinking it would be?
Star forts had a primary armament of cannons, to be able to fire both at enemy siege artillery and at aproaching enemy troops, preferably with shrapnel or grape shot.
The Swedish fortress of Sveaborg carried 734 cannons and mortars when it fell to the Russians (mostly through a ruse) in 1809. There were enormous supplies of shot and gunpowder to be able to keep firing throughout a siege.
The trenches built were assault trenches, that zigzaged so that a cannonball could not run the length of a trench. The trenches would be digged close enough for the troops to assault the walls of the fort - preferably at a breach created by siege artillery.
The besiegers would built earth redoubts for their own siege artillery, and unless they managed to bring heavier siege artillery than the fort carried, they would be within each others range and continously duel, trying to destroy each other.