Fandyllic's advice is solid. I would add a good looking and readable color pallet is another must. The transparent background makes it harder to read text, so I'd go with a solid one. Since your topic is more encyclopedic than sandboxy, I'd get rid of the squidward artwork on the main page, it doesn't really make the site look like a serious source but more of a joke site.
I would also look into learning HTML and CSS to help refine the look of the site. It's a trend in newer wikis, typically managed by younger people I've noticed, to go overboard with CSS and over design the wiki to the point where it's nearly unreadable with lots of effects and scripts, so I'd avoid that. As the old saying goes, keep it simple, stupid. But still, theirs lots of roughness on the main page that needs ironing out.
The header navigation is also really rough and hard to navigate with. Refine that and it'll make your site easier to use and more professional looking - which is key in a wiki like yours. For example, when you hover over a button, they still say "category" in it and have underscores.
Your titles for articles are also not really SEO friendly, or user friendly. The article on the American Revolutionary War is just called "The Revolutionary War", ignoring the fact that their were several other revolutionary wars all over the world prior, and after.
But aside from all that, the most important step is to have quality content. If you use all the SEO tricks in the book but have lackluster content, you might get some traffic, but your site isn't going to be taken seriously, at least in the academic circle (lord knows crappy history sources have been all the rage lately). Look for relatable sources, don't fall for baloney sources like pseudohistory documents and for the love of god, don't rely on videos or documentaries. Cite actual peer reviewed works and primary and secondary sources. Don't be afraid to ask around on history forums like AskHistorians for help regarding a specific subject matter and always check the sources you're given.
Specialized coverage into specific events not really covered by other sites will likely help in SEO too. If theirs a specific period or event you're very much interested in, work on creating quality pages for that subject.
Lastly, lite advertisement can be useful in getting something off the ground. Don't spam it, rave about it, or annoy people with it, but link your sites content if relevant. Putting it in your profile description or signature on history forums and websites can also be a useful tool to give it more exposure without spamming.
But all that said, the unfortunate fact is theirs no guarantee your wiki will ever get big. Just work hard, always strive to improve it and expand it and you might find your audience eventually.