You might have known that crenellation is the uppy downy bits on battlements. You can have a point if you did. To crenallate is to provide a castle or tower with crenellation.
Did you know the uppy downy parts also have proper names? The gaps are crenels or crenelles. The bits between are merlons. Here's a sign to prove I don't just make all this stuff up.
We got there early and had the place to ourselves for the first half hour. I really must work it into a story.