Looking at the thread of Amazons vs. Elves, I've always pondered why the LOTR type universes were frozen in technology for thousands of years at basically Middle Ages tech (excluding magic). There's an interesting interest of how tech took off in Europe while Asia and the Middle East lagged behind.
In any case, let's assume that about 100 or so years before the LoTR quest, the Gondor and Rohan folks have a tech breakthrough in weapons design and come up with musketry - let's say BP, Minie balls, rifles and similar flintlock handguns. No primer fired weapons. They equip their infantry and cavalry with such. How does this change the battles of LoTR, attack on Gondor, Helm's Deep, March to Mordor.
Note, I haven't added artillery. If you want as a second path, add such and mobile guns such as Gustavus Adolphus used for the Swedish Armies.
Sauron had explosive but in general that tech was not used much.