If the defenders had such good fortifications as has been previously described, how could they lose? It goes to follow, if the besieged were consistently victorious, sieges would have become few and far between because - why bother - there would have been near guarantee of victory for the defenders. But during this era there was one siege after another, from the Mediterranean to the Baltic. Sieges were a fixture of war - and many fortifications fell. Obviously, something was not working for the defense.
Were the numbers of soldiers and their weapons in rough balance for the defense and the attack, the defense could likely have won a siege. But defenders were usually outnumbered in men and cannons, and often by a considerable margin.
Why the difference in numbers?
Soldiers had to be assigned to the fortifications - after all, no fort worked unless staffed (recall that tired cliché of modern corporations: "People are our most important asset" - soldiers were the most important asset for a fort). Nevertheless, an army still had to have sufficient numbers remaining in the field force to make battle and to lay sieges of their own against the enemy. Furthermore, a field force proved to be a vital asset for defense: a force to come to the aid of a beleagured fortification. Not surprising, a commander or governor or ruler tried to have as many of his soldiers remain part of the field army - as a single entity - and this often distilled down to around half the total of his forces.
In contrast, that huge portion delegated for garrison duty was rarely kept together in one fortification or city. Its numbers were divided, sometimes quite finely. The Army of Flanders in 1639 garrisoned 33,399 of its soldiers in 208 locations. Thirteen years earlier that same army committed 31,046 men to garrison duty out of a total of 48 - 55,000. Cosimo del Monte, as senior military advisor for Venice, advised in 1621 that half the forces he proposed to protect mainland territories go to garrisons while the other half "serve in campaigns" and "assist in the defense of the cities."
Mind that not all garrisons were assigned to fortifications. Some simply settled into small towns to be an occupying force instead of a barrier to enemy incursion.
Any garrison did more than simply "hold down the fort." It also had to control the surrounding countryside and its civilian population. If the enemy had its own garrison not far away, the two little forces took whacks at each other without even going into a siege. This was an aspect of "kleine krieg," which will be discussed soon.
In order to equip their field armies sufficiently to be effective, at times commanders requisitioned arms and men from their own forts. Artillery in particular was taken for campaign use. Too often defending artillery was then no match to that of the attackers in caliber or numbers:
Even with a goodly number of men to defend, a fortification could still have, um, 'situations.' For example:
Despite 20,000 men defending Nicosia when besieged by Turks in 1570, only half were considered effective due to poor training and disease. On top of that, 900 arquebuses were sent outside the fortifications before the siege began and thus were of no use in the defense, but that may not have been such a loss because "the soldiers did not know how to discharge them without setting fire to their beards." In addition (yes, there's yet another fumble), the crops that grew between Nicosia and Famagusta were a point of contention between the two cities. Because the dispute over who should cut and store those crops was still unresolved when the Turks showed up, those Turks took that food still in the fields. Oh, and Nicosia's commander restricted powder distribution and forbade his troops to shoot unless the Turks presented themselves in groups of ten or more. After knocking over Nicosia in a mere seven weeks, the Turks turned on Famagusta, which held out 11 months.
The defense of a fortification suffered one more disadvantage.