What are we looking at it?
It’s basically a wrought-iron gate like this one:

The way Doyle describes it, the gate design is so complicated that it is:
A maze wrought in iron
No more than that, it’s:
A maze of fantastic tracery wrought in iron
That’s actually a complex image so let’s build our own version in two stages.
Let’s go back to the simplified version:
A maze wrought in iron
The gate is made from wrought iron, so that is literally true. However, the word maze is a metaphor.
So we have this:
A maze wrought in iron
Metaphor material
Let’s try a few variations. To do this, start with your material, then add a metaphor.