The introduction of cannons on the battlefields of the late Middle Ages changed much, particularly regarding techniques of fortification.
The rather weak walls and city gates, normally sufficient to defend against infantry, did not last against cannon balls made of stone and iron.
This is also what the defenders of Constantinople experienced. In 1453, a hail of cannon balls weighing over one hundred kilograms breached the city's famous double wall ring, in order for the Janissaries of Sultan Mehmet II to enter. Constantinople fell to the Ottoman Empire.
In Europe, attempts were made to protect castles and fortifications with semi-circular artillery towers. In the medieval city fortification of Neubrandenburg (Mecklenburg-Vorpommern), which has almost completely been preserved, the Friedländer Double Gate is guarded by a rampart with thick walls and cannons.
A further development consisted of shorter round towers with fighting platforms. The cannons erected there were able to fire in all directions, as opposed to those of the ramparts proper.
However, even these could not stop the effectiveness of heavy firearms, although the modern star-shaped bastions with earth walls - the remnants of which can still be found in various cities - would constitute a definite improvement.
Translation: Erik Eising (MA)