Crenellations, or notches were added to the tops of towers in order to provide stone shields for more effective defense. So that correct option is (b).
A defensive architectural element called crenellations is most frequently seen on the battlements of mediaeval castles. A battlement is a small parapet used for defense. Cutting crenels into a parapet wall that was once solid and straight is known as crenellations. Regularly spaced throughout the parapet, crenels are rectangular openings or indentations that are typically 2-3 feet wide. Merlons, which typically measure 4-5 feet wide and 3-7 feet high, are the solid widths in between crenels. With cover behind the merlons, arrows and other missiles may be fired from the parapet through crenels. Crenellations are often a sign that a castle or other structure, like a manor home, was built with defense in mind against a fight.
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