Couple of points ...
Firstly, depending on how advanced the muskets are (English Civil War-era, Napoleonic Wars, American Civil War?), those shields might well stop a musket ball.
Secondly, what's the premise? The western Roman Empire surviving an extra 1,200 years into the gunpowder age, or gunpowdwer weapons being developed in 200BC-ish? (going by the elephants in that one pic, I presume it's Hannibal's campaign in the 2nd Punic War)? Or is it a general quasi-historical-fantasy-steampunk mashup (with Carthaginians, early Imperial Roman legions, Samurai and Mongols, I presume the latter)?
Oh, and I should have said, the art is exceedingly cool in any case, and I'm not nearly as critical as this post sounds. Keep us informed of the book's progress, I may have to buy a copy
