There's a fair bit of wriggle room in the term "history-based", but the additions to free-to-play MMORTS Terra Militaris may just have popped some sort of reality seam, flooding the pre-industrial world with guns and monstrous house-sized pets. All for the better, I say: the new Firearms expansion not only brings a satisfactory quota of splodes, it also overhauls a good deal of the game.
Terra Militaris is a free-to-play, browser based massively multiplayer RTS from gPotato. You build an empire by developing and upgrading your city, exploiting local resources to build larger armies. Once you're strong enough you can venture further afield and start meeting up with other players, forming alliances and waging war on your neighbours for their treasure.