I was reading a biography of sultan Suleyman and got curious about how the cannons of the time (early-mid 16th century) functioned in battle. There was a description of some of the absurdly large cannons the Ottomans had, cannonballs with a circumference of 2,5 metres and such. So clearly they were very powerful, some of the most powerful in the world; but still sieges were slow and brutal affairs, taking at least weeks and often months. So I came to think what was it exactly that prevented these huge cannons from tearing down walls faster? I thought of a few things:
That (in this specific case) places like Belgrade, Rhodes and Vienna already had highly developed star forts that could withstand many solid cannonball shots
That the structure of the cities and star forts was such that the besiegers could not get close enough to hit the walls at an efficient rate, what with vast trenches, banks and defensive fire from inside the fortresses
That aiming with the cannons was simply a crapshoot, and with how labourous they were to load, you would be lucky if one or two cannonballs a day hit the walls, let alone a critical spot
I assume the real reason is a combination of these three (if they're true, which is really the "few questions" bit) and also additional factors, so I'm curious what else played into it. Cheers.
All those reasons are correct. Star forts, or trace italienne are very difficult to tear down. They limit or eliminate possible safe angles of approach by an attacker, and because of their construction, they remain effective even if you manage to punch holes on a wall.
What tends to happen in the siege is that it turns into multiple battles, direct and indirect, against each other. The attacker will try to dig trenches to come closer to the fort while the defenders will fire on the trenches. The attacker will try to dig tunnels and set off bombs to weaken some structures. The defenders dig counter-tunnels. Aiming those big cannons is also crap shoot.
But you miss one important reason why the cannons fired so slowly: they needed time to cool down, and for both the guns and their emplacements to be maintained in between shots. Just the cool down could take up to an hour for a very large bombard. After that the guns were cleaned, inspected, and its cracks were to be patched up as much as possible. The emplacements, often wooden, had to be patched up, too. Failure to clean and patch means risking explosion at worst, less power at best.
Of course, the pressures of war often forced cannons to be fired faster than they were cooled, something that was obviously very risky for the gunners. In sieges, defenders often keep track of how fast the attackers' guns are fired, to gauge their level of desperation.
There is a magnificent description of early modern sieges of Crete, and of Malta, in Roger Crowley, "Empires of the Sea: The Siege of Malta, the Battle of Lepanto, and the Contest for the Center of the World," ISBN 978-1-58836-733-4, 2008.