Which architectural feature consists of a series of regular, rectangular spaces cut out of the top of a wall?

The architectural feature described in the question as a series of regular, rectangular spaces cut out of the top of a wall is known as crenellation. Crenellation is typically characterized by a pattern of alternating solid and void forms, often seen on battlements and parapets. These cut-out sections create a defensive architectural element that provides protection for those inside the structure while allowing them to observe the surroundings.

This design element is commonly associated with medieval castles but can also be found in various forms of architecture throughout history. It serves both a functional and aesthetic purpose, contributing to the distinctive silhouette seen in many fortified buildings. Understanding crenellation helps in recognizing how architectural features historically adapt to defensive and stylistic needs in various contexts.