Dumas's syntax is simple and straightforward, which leaves only the occasional and easy to remedy vocabulary problem.

I agree with Legolas. Castelot's French is more "academic", a bit more old fashioned (not that much, native speakers are a bit puzzled when people find French has changed a lot since the 19th century. Spoken French more, but written French has barely changed, aside from literary trends), but it's fairly standard, with good grammar, and nothing too fancy or elaborate syntax wise. It's easier than most fiction literary prose, I would say (especially since having read some Dumas and Bertière you're no doubt versed in the vocabulary of things/titles/concepts from those times, not to mention you're even more familiar with the English vocabulary related to this period and you'll be able to guess the meaning of many words from that).