You are right to be wary. An empty metaphor is a missed opportunity, and missing your opportunities is bad style. Please note that this doesn't mean this is a "bad" metaphor. It means that it's a metaphor that isn't (currently) working. In the right context, it could be a great metaphor. In fact, it might be leading you in a productive direction, one you just need to expand upon.
I torch the forests with a sword of flame, and top the trees with a crown of fire.
Now this metaphor is doing work. It's telling you things about the narrator (the evil king you suggested). Here's a different example:
The regal forests were robed in green, but crowned with fire.
In this case, it's the forest itself that's personified as royal. These usages are easy to understand, because there's some context letting you know what connotations of crown are important. In your original example, the reader has to work hard to guess what you mean. A crown is a circle, it has jagged points, it goes on the top or the head, it signifies a monarch. So which of those is applicable here? If you're just going with the simple, geometric answer, you're right, you're not gaining anything over the more direct "circle." In fact, you've made your job harder because crowns don't expand, and a circle of fire does.