Fantasy Hack is an old school mass combat game which fits right in with D&D, Lewis and/or Tolkien settings. If you are into OSR, this could be your go to mass combat set.
The rules are totally retro feeling because they are printed in one color, black, on yellow paper. Published in 1993, they'll send you back a couple of decades.
While not perfect for every setting, the set manages to handle most fantasy settings. Give it a try.