Also the movement from square towers to round ones, once again as a defense for sapping, and displacing the tower from the wall or keep in order to be able to flank assaulters attempting an escalade.
The mastery of stone fortifications made walls nearly impregnable to breaches until cannons were developed, and even by then it was a dodgy proposiiton. For instance at Constantinople in 1453 the Turks had huge cannon which could damage the walls, yet it was not the reason the city fell. The defenders were able to repair the walls before the cannon could reload for a second shot. The reason Constantinople fell was a side gate was left open (either intentionally or accidentally) which allowed the Turks to pour through. There were alot of incidents like this such as the defenders of the Krak de Chevaliers (in modern day Syria) being duped into believing their commander had ordered their surrender
I thought the move to round towers was due to the increased effectiveness of canon.