"Crenellation" is a synonym for "castellation". Both words refer to the battlements or fortified walls often found on castles or fortresses. Another synonym is "embattlement", which also pertains to the protective walls used in defensive architecture. "Parapet" is another word that can be used interchangeably with "castellation", as it refers to a low wall or railing that runs along the edge of a roof or balcony. "Merlon" is also a synonym for castellation, specifically referring to the solid upright sections of a parapet, between the crenels. Overall, these synonyms each provide alternative ways to describe the fortifications built to protect important structures.