One of the first battles to actually use gunpowder in Europe was the siege of Granada (the city, not the island country that was named after this city) in 1491. At that time, gunpowder cannons were pretty much equivalent to other siege artillery weapons of the era like a Trebuchet, Ballista, and Catapult both in terms of reload speed, accuracy, and actual damage performed. If anything, the gunpowder based wepons of the era were even harder to use, required special equipment that was even heavier to move, and needed more maintenance.

As a side note, that particular battle in Grenada happened to have Christopher Columbus present as he was trying to convince the King of Spain to help finance his trip across the Atlantic Ocean.

I would imagine that without gunpowder, refinements in those other forms of artillery would have continued with further refinements in accuracy from mechanical attacking devices instead. Perhaps the Archemedes Mirror might have been finally deployed as well, or a variety of other similar siege and anti-siege equipment.

No doubt fortifications would have changed as a result. Things like star forts were built explicitly because of gunpowder artillery, and of course you have the Maginot Line, the German equivalent with the Siegfried Line, and the coastal fortresses like was found on Omaha Beach that ended up being a major obstacle during D-Day. Fortresses definitely didn't end with the advent of artillery, and of course the battle of Monte Cassino shows how even in modern warfare those mountain toppoed fortresses can still cause brutal damage against an attacker. I doubt even the use of nuclear weapons would have done more in that particular battle other than increase the already high casualty rates of both sides.

I agree that artillery is a major component of warfare and something that strongly influenced the development of gunpowder weapons, but it is far too easy to overestimate just how much gunpowder in at least the 13th through 17th Centuries might have changed things in warfare when there were other competing technologies of the era.