Dismantling Fortifications
As the power of cannons grew during the 16th and 17th century, medieval walls became obsolete as they were too thin to offer any realistic protection against prolonged bombardment. As a consequence of this, many walls from medieval times were torn down and the stone (still valuable as construction material) reused in more modern bulwarks and bastions. The resulting space is often seen in old city centers of Europe even to this day, as broader streets often outline where the old wall once stood (evident for example in Prague and Florence, Italy).
Read more about this topic: Medieval Fortification