To fortify a wall with battlements (holes or notches used for shooting at an approaching enemy) is to crenellate it. If you crenellate the cardboard towers, your model of a medieval castle will look more realistic.
Imagine a protective wall surrounding a castle, and the notches cut in that wall that can be used to watch for approaching troops and, if necessary, fire at them. The notches themselves are called crenels or embrasures, and the process of installing them is crenellating. The word stems from the Latin cren (also the root of "cranny"), which means "notch."
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