The word "crenelate" means to provide a fortification with crenellations or battlements. The antonymic words of "crenelate" are "smooth," "level," "flatten," and "refine." These words all go against the idea of creating a fortified structure as they denote a process of making something flat, level, and without any rough edges or indentations. So, if you are seeking to describe a surface as the opposite of "crenelated," you could use any of these antonyms. For instance, instead of using "crenelate" to describe a wall or castle, you could use "smooth," "level," "flatten," or "refine" to convey the opposite impression.