Sometimes I think my metaphors require more research than the rest of my novels' worldbuilding put together. It can be tricky, in a first draft, to hit a metaphor which is in keeping with the worldbuilding but at the same time not so unwieldy that a modern reader is going to stumble over it. Yesterday I spent a good twenty minutes researching the history of barbed wire (invented in the 1860's, in case you're curious, so barbed wire itself was out) and chasing mentions of the use of "thorny brush" as a fencing/deterrent which could have provided an analogous metaphor … only to realise that the simple fish hook, which has been around since time immemorial and requires no fancy descriptions to be understood by any reader, was a far more apt metaphor for the situation.
Um, yeah. This is how I spend my time. Willingly.